the definition of life

8/10/2019 The Definition of Life

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http://www.baharna.com/philos/life.htm

The Definition of LifeBy Joseph Morales

The aim and the end of all becoming is the development of potentiality to actuality, theincorporation of form in matter. Aristotle, attributed by Jeremy Campbell, in

Grammatical Man

And it is in no way possible for anything to be responsible for its own generation anddecay. For the mover must preexist the moved, and the begetter the begotten. But nothing

is prior to itself. Aristotle,De Motu Animalium, trans. Martha Craven Nussbaum

The cause of the origin of a thing, and its eventual utility, its actual employment and

place in a system of purposes, lie worlds apart.Friedrich Nietzsche, quoted in Daniel C.

Dennett,Darwins Dangerous Idea

Contents

Defining Life

Criteria for Our Definition

Simple Replication

Von Neumann Replication Resistance to Entropy

Autopoiesis

Responsiveness

Continuance Through Others

Homeostasis

Patterns of Complexity

o Algorithmic Complexity

o Logical Depth

o Effective Complexity

o

Ranges of Fluctuation Behavior

A Matter of Degree


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Defining Life

Since life is such a ubiquitous and fundamental concept, the definitions of it are legion.

John D. Barrow and Frank J. Tipler, The Anthropic Cosmological Principle

Words are our servants, not our masters. For different purposes we find it convenient touse words in different senses. Richard Dawkins, The Blind Watchmaker

What is life? And why should we care? Well to begin with, we are living beings, and that

fact distinguishes us from most things in the Universe. Though humans are not the onlyliving things, we are among the few, so understanding the nature of life might be an

important step toward understanding ourselves.

As Richard Dawkins points out, people define life in different ways for differentpurposes. For everyday situations, it seems to me that we have a common-sense set of

criteria, somewhat along the lines of:

Does it look like a person or an animal? If so, is it moving? Does it respond to being

spoken to or touched? Failing this, is it breathing, or is its heart beating?

Does it look like a plant? If so, does it have green leaves? If not, could it be because it is

winter? . . . Etc., etc.

The question is, can anything meaningfully be done to define life that would not simply

be a repetition of such everyday notions. Once upon a time, philosophers like Platobelieved that things in the everyday world are imperfect reflections of perfect forms or

concepts in some type of higher realm. In defining a class of objects, we would be

searching for an understanding of that mystical essence that inheres in all of them.

But as David Hume said, somewhat more recently,

Nothing is more usual than for philosophers to encroach on the province of grammarians,and to engage in disputes of words, while they imagine they are handling controversies of

the deepest importance and concern.

Even more recently, the logical positivists have stressed the distinction between analytic

and synthetic propositions. An analytic proposition is one that is true by definition, suchas saying that "men are adult male humans." Such a statement is an assertion about

words. On the other hand, a synthetic proposition asserts some new truth about the worldthat was not inherent in the words themselves. Synthetic propositions are subject toverifiability; we can perform experiments or other observations to determine whether

these propositions are true.

A third class of propositions consists of statements that are neither analytic or synthetic,and to the logical positivist, such statements are simply without sense. The logical


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positivists sought to expose much of traditional philosophy as meaningless discourse, a

sort of neurotic disease.

Which of these three types of statementsanalytic, synthetic, or meaninglessam I

planning to make about life? Well, neither of the first two. Whether my statements will

fall into the third category remains to be seen.

I would argue that it would be a good thing to have a workable abstract definition of life,

and that such a definition need not be wholly arbitrary, but can be defensible to a degree.However, the point is not to describe some sort of metaphysical essence of life. Rather,

the point is to define life so that the term can be usefully extended to situations we have

never before encountered.

For example, the legal definition of "death" now has to take account of situations thathave only recently been made possible by medical science. Nowadays many comatose

people can be kept on life support machines for years. But are we to regard this as life, or

specifically as human life, with all the ethical considerations that we attach to humanlife?

But the issues reach beyond ethics and into the area of scientific discourse. Researchers

such as James Lovelock have recently developed the Gaia theory, which is the notion thatthe earth as a whole is a living organism. The idea is vigorously disputed by many other

scientists. But the real question is, how scientists can even have a debate about whether

the earth is alive, if they havent agreed in advance what they mean by "alive"? Lovelock,

to his credit, does make an effort to define his concept of life, but states his conclusions

in only the most incomplete and tentative sort of way.

The idea of Gaia is said to have been sown in popular culture by the photographs ofgreen-and-white Earth taken from the barren moon. Similar changes of perspective mayresult by our explorations of other planets, such as Mars. If we found life there, could we

recognize it? After first noticing it, could we agree on whether it was indeed life or not?

The possibility of meeting extraterrestrial life may seem a distant one. Lovelock himselfhas provided good reasons for believing that there is no life currently on Mars, whether or

not there was in the past. And the density with which life is scattered through the galaxy

or the universe as a whole is almost totally unknown. We might make contact tomorrow

or never. (Sometime in between would be my guess!)

But there are two much more immediate trends on our own planet that are forcing us tostretch and readjust our definition of life. These are trends in biotechnology and

information technology.

The Human Genome Project is currently mapping the locations and function of all the

genes in human beings. It is difficult to fully appreciate the significance of this project.With such information in hand, or even a small part of it, we will attain the ability to

redefine ourselves in ways that we have not yet begun to imagine. Clearly, we will need


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order to be called "living." We shall abstract these sufficient conditions from the various

definitions proposed over the last thirty years by biologists. John D. Barrow and Frank

J. Tipler, The Anthropic Cosmological Principle

Whenever biologists try to formulate definitions of life, they are troubled by the

following: a virus; a growing crystal; Penroses tiles; a mule; a dead body of somethingthat wasindisputably alive; an extraterrestrial creature whose biochemistry is not basedon carbon; an intelligent computer or robot. William Poundstone, The Recursive

Universe

We will be searching for a definition of life that is useful. In order to be useful, the

definition should meet the following criteria, so far as possible:

Sufficiency. It should provide the sufficient conditions that enable us to specify whether

something is living or not.

Common Usage. These conditions, when applied to "easy" examples, should classifythose examples in the same way we normally do. Easy examples include obviously livingthings such as people, animals, plants, and bacteria; things that were alive but are now

dead; and things that we would never normally consider alive, such as rocks,

screwdrivers, and growing crystals.

Extensibility. It should be possible to apply these conditions to "difficult" examples with

some kind of coherent result. Difficult examples include viruses, mules, fire, simple

feedback systems (such as those with thermostats), Gaia, extraterrestrial creatures, and

robots.

Simplicity. The definition should be as simple as possible, with a minimum of ifs, ands,or buts.

Objectivity. The definition should refer to measurable and objective properties of the

organism. That is, the definition should be specific enough so that different people can be

counted on to apply the definition in the same way when they encounter a new "difficult"

example.

Once we have determined the sufficientconditions that something must satisfy in order to

be considered living, we can go on to ask what additional properties follow from the factthat something is alive. Life may involve certain engineering problems that have only a

finite number of possible solutions. Other engineering problems may have a variety ofpossible solutions, including many that have not been used by any familiar life forms.

Our procedure will be to review some of the definitions that other authors have given,

beginning with the most naive and progressing to the more satisfactory. Then we will try

to improve on the best existing definitions.


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Simple Replication

Virtually all authors who have considered life from the point of view of molecularbiology have regarded the property of self-reproductionas the most fundamental aspect

of a living organism. John D. Barrow and Frank J. Tipler, The Anthropic Cosmological

Principle

Barrow and Tipler thus begin their discussion of life with a statement that is historically

untrue, as we shall see later; not all molecular biologists regard self-reproduction as themost fundamental or defining aspect of life. And by beginning this way, Barrow and

Tipler commit themselves to a particular type of definition without stopping to consider

whether it is really necessary.

Almost immediately this definition runs into trouble, because on the one hand, there are

easy examples of living things that do not or cannot reproduce; and on the other hand,

there are easy examples of non-living things that do reproduce.

Barrow and Tipler are aware of these difficulties, and they begin by discussing the livingthings that do not self-reproduce: among them, childless people and mules. (Mules, of

course, are the offspring of horses and donkeys, and cannot have offspring themselves.)

Then the authors present the following rationale:

But such creatures are metazoans, which means that they are all composed of many

single living cells, and generally each cell is itself capable of self-reproduction. Many

human cells, for instance, will reproduce both in the human body and in the laboratory. Ingeneral, allknown forms of living creatures contain as sub-structure cells which can self-

reproduce, or the living creatures are themselves self-reproducing single cells. All

organisms with which we are familiar must contain such cells in order to be able to repairdamage, and some damage is bound to occur to every living thing . . . The ability to self-

repair is absolutely essential to a living body.

Since all living things are largely composed of cells which can self-reproduce, or are

autonomous single cells with self-reproductive capacity, we will say that self-reproduction is a necessary property which all living things must have at least in some of

their substructure.

What the authors have done at this point is to jump to a different level of analysis. At the

start, they were talking about self-reproduction of the organism as a whole, but now they

are settling for self-reproduction of the constituent parts. The strangeness of thisviewpoint is evident if you consider the following example:

"He La" cellsfrom the cervix of Henrietta Lane, a woman who lived in Washington,

D.C.continue to be grown in laboratories around the world, despite Lanes death fromcancer of that same cervix in the 1950s. Lynn Margulis and Dorion Sagan, What is

Life?


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Now, this is an easy example of someone who is not living, but whose constituent parts

are continuing to reproducecontinuing for decades, in fact. It is true that HeLa cells arestill alive, but Henrietta Lane is not. This fact suggests that one ought not to confuse the

properties of an organism as a whole with the properties of the parts that constitute it.

Barrow and Tipler come up with a similarly awkward rationale for dealing with thethings that reproduce but are not alive. They mention the examples of salt crystals and

mesons, each of which will produce copies of itself under suitable conditions, and state:

Yet we would be unwilling to regard either salt crystals or mesons as living creatures.

The key distinction between self-reproducing living cells and self-reproducing crystalsand mesons is the fact that the reproductive apparatus of the cell stores information, and

the specific information stored is preserved by natural selection. The reproductive

"apparatus" of crystals and mesons can in some cases store information, but thisinformation is not preserved by natural selection . . . Ultimately, it is natural selection that

corrects errors and holds a self-reproductive process together, as Eigen and Schuster have

shown in their investigation of the simplest possible molecular systems exhibiting self-reproduction. Thus, basically we define life to be self-reproduction with error correction.

There are some oddities about the new clause in their definition. First, natural selectiondoesnt necessarily perform error correction, although that may be its effect much of the

time. Natural selection preserves the organisms best adapted to their environment, not

those most similar to their parents. The distinction may not be important in the case of

most mutations, which are distinctly negative in their effects (unless their effect ismasked by the corresponding gene from the other parent). But if natural selection always

worked to perform error correction, evolution would not be possible.

Moreover, there is a more serious objection. Barrow and Tipler have made another jumphere to a different type of analysis. Suddenly life is being defined based on a historical

criteria. In other words, something is being judged as life not on the basis of what it is or

how it behaves, but on the basis of the way it was created. This is a troublingdevelopment, because it means that we could not tell whether something is alive unless

we also know that it is the product of Darwinian evolution. But people knew that things

were alive before Darwin ever proposed his theory!

Additionally, this criteria would exclude life forms created by design. Surely, if we had

been created by an intelligent God, it would not follow that we were any less alive?

Following are some of the applications that authors draw from this definition:

Note that a single human being does not satisfy the above sufficient condition to be

considered living, but it is made up of cells some of which do satisfy it. A malefemalepair would collectively be a system capable of self-reproduction, and so this system

would satisfy the sufficient condition. In any biosphere we can imagine, somesystems

contained therein would satisfy it . . .


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A virus satisfies the above sufficient condition, and so we consider it a living organism . .

.

Automobiles, for example, must be considered alive since they contain a great deal of

information, and they can self-reproduce in the sense that there are human mechanics

who can make a copy of the automobile. These mechanics are to automobiles what aliving cells biochemical machinery is to a virus. The form of automobiles in theenvironment is preserved by natural selection: there is a fierce struggle for existence

going on between various "races" of automobiles! In America, Japanese automobiles are

competing with native American automobiles for scarce resourcesmoney paid to themanufacturerthat will result in either more American or more Japanese automobiles

being built!

At least the authors are applying their definition consistently. But in doing so, they haveshown that it cannot handle the easy examples. For surely a single human being is an

easy example of something that is alive, and an automobile is an easy example of

something that is not alive. A virus, on the other hand, is a difficult example that isconsidered a living thing by some biologists but not by others; it is a borderline case at

best.

To summarize, the Barrow/Tipler definition of life fails two of the criteria we proposed at

the start: it doesnt handle the easy cases, and it isnt simple (they have to add special

clauses to handle cases like mules and human reproduction).

How did the authors come to settle on a definition with such obvious inadequacies? Werethey using different criteria than us for developing their definition? It seems not. Rather,

they are aware of the problems with their definition, but they have despaired of finding

anything better:

A consequence of giving sufficient conditions rather than necessary conditions is the

elimination from consideration as "living" many forms of matter which most people

would regard as unquestionably living matter. This situation seems unavoidable inbiology. Any attempt to define some of the most important biological concepts results

either in a definition with so many caveats that it becomes completely unusable, or else in

a definition possessing occasional ambiguities.

Let us proceed through some of the alternatives and see whether we can come up with

anything more satisfactory.

Von Neumann Replication

The writer William Poundstone, in a fascinating book called The Recursive Universe,

advocates a definition of life based on some work by the great Hungarian/American

mathematician John Von Neumann.


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Von Neumann was much occupied by the question of how an organism can reproduce

itself. But not all types of reproduction were equally interesting. Von Neumann drew adistinction between trivial and nontrivial self-reproduction. We have already encountered

some examples of trivial reproduction in the previous section: salt crystals and mesons.

Poundstone mentions some further examples, including certain types of tiles invented by

geneticist L. S. Penrose. (These are not to be confused with the "Penrose tiles" inventedhis mathematician/physicist son, Roger Penrose.) If you have a tray full of these tiles, and

you shake the tray so they jostle together, they remain a disorganized mess. However, if

you link two of the tiles together in a particular way, and shake the tray again, the othertiles start linking together into pairs that mimic that first seed pair. The first "seed" pair

can be said to have reproduced.

Remember that Barrow and Tipler added their criterion of "preserved by naturalselection" to exclude such trivial examples of self-replication. Von Neumann took a

different tack by explicitly defining what he would consider as non-trivialreproduction.

To Von Neumann, non-trivial reproduction must involve a "universal constructor," which

is a mechanism that can create any number of different things, provided that it has ablueprint to work from.

In the following quote, Poundstone derives the following criteria for living things from

Von Neumanns work:

(1) A living system encapsulates a complete description of itself.

(2) It avoids the paradox seemingly inherent in (1) by not trying to include a description

ofthe description inthe description.

(3) Instead, the description serves a dual role. It is a codeddescription of the rest of thesystem. At the same time, it is a sort of working model (which need not be decoded) of

itself.

(4) Part of the system, a supervisory unit, "knows" about the dual role of the description

and makes sure that the description is interpreted both ways during reproduction.

(5) Another part of the system, a universal constructor, can build any of a large class of

objectsincluding the living system itselfprovided that it is given the proper

directions.

(6) Reproduction occurs when the supervisory system instructs the universal constructorto build a new copy of the system, including a description.

The Recursive Universe, Chapter 11

Together with Arthur W. Burks, Von Neumann proved that such "self-reproducingautomata" are possible. This theory also proved to be a correct, though abstract, model of

how real cells reproduce using DNA. So it seems clear that Von Neumann did hit upon


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something important about the nature of reproduction. The question is whether this model

also amounts to a definition of life.

This definition does seem more promising than the Barrow/Tipler definition, in that Von

Neumanns criteria for non-trivial reproduction are functionally based rather than

historically based. How does it fare when applied to easy and difficult examples?

The Von Neumann criteria do eliminate some easy examples of non-life, including salt

crystals, mesons, and Penrose tiles. These criteria go further than Barrow/Tipler by alsoeliminating automobiles (because they not include universal constructors). So this

definition deals well with some easy examples of non-life.

The definition also gives an interesting answer for one of the difficult examples. Viruses

are considered non-living because they also do not include universal constructors;instead, they invade cells and commandeer the universal constructors there in order to

reproduce. Thus viruses meet only the first three out of the six Von Neumann criteria.

This seems to be an acceptable conclusion for what is usually regarded as a borderlineexample of life at best.

Unfortunately, the definition does not deal so well with some of the other easy examples.

Under this definition, mules would clearly not be living. Poundstone denies this, but byusing the same rationale that Barrow/Tipler adopt: he refers to the fact that the mule is

made of cells that themselves reproduce. As we have seen before, all that really follows

from this observation is that a mule is a dead thing made out of living cells. Thus, I

mentioned previously the example of HeLa cells, the living cells of a deceased person.

In fact, in the next paragraph Poundstone asserts that dead bodies in general should be

considered living, at least until decay destroys the internal structures of the cells. For,until that happens, it is conceivable that some bioengineer might clone a new person froma cell of the dead person. Thus, the dead person has reproductive potential, and should be

considered living.

If we were discussing a less nebulous concept, the "dead people are actually living" resultcould be taken as a sort of reductio ad absurdum, a proof that this definition must be

wrong because it leads to impossible conclusions. However, for now we will simply state

that this seems a disappointing result, and that it seems profitable to search further for a

definition before giving up.

Resistance to Entropy

What is the characteristic feature of life? When is a piece of matter said to be alive?When it goes on "doing something," moving, exchanging material with its environment,

and so forth, and that for a much longer period than we would expect an inanimate piece

of matter to "keep going" under similar circumstances. When a system that is not alive is


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isolated or placed in a uniform environment, all motion usually comes to a standstill very

soon as a result of various kinds of friction; differences of electric or chemical potentialare equalized, substances which tend to form a chemical compound do so, temperature

becomes uniform by heat conduction. After that the whole system fades away into a dead,

inert lump of matter. A permanent state is reached, in which no observable events occur.

The physicist calls this the state of thermodynamical equilibrium, or of "maximumentropy." Erwin Schrodinger, What is Life?

As a physicist, Erwin Schrodinger was in a good position to appreciate the oddity of life.

A few decades before his time, the second law of thermodynamics had been formulatedin its statistical form by Ludwig Boltzmann. According to this "entropy" law, any closed

system moves inexorably toward a state of increasing disorder. How are living things

able to postpone the inevitable for so long?

Schrodinger proposed an answer to that question, but before we get to it, let us consider

whether the characteristic proposed above is a sufficient definition of life. Note that the

criterion of self-replication, central to the Barrow/Tipler and Von Neumann definitions, isnot even mentioned. We have at this point encountered the first of a new category ofdefinitions: those that emphasize the ongoing processes of life rather than its reproductive

potential.

Barrow and Tipler have the following to say against such definitions:

But if information preserving (or increasing) reproduction is removed from the list of

physiological processes, then it seems that candle flames must be considered livingorganisms. Flames "eat" or rather take in fuel such as candle tallow, and they "breathe"

oxygen just as animals do. The oxygen and fuel are metabolized (or rather burned) in a

reaction that is essentially the same as the underlying oxidation reaction that supplieshumans with their energy. Flames can also grow, and if the fuel is available in various

nearby localities, move from place to place. They can even "reproduce" by spreading.

Is flame alive by Schrodingers definition? In other words, do flames postpone the stateof maximum entropy much longer than we would expect a non-living thing to? Well,

there is something self-sustaining about flames, in the sense that, as the fuel burns, new

fuel is continually being exposed and heated to the point where it can also burn.

Schrodingers definition, by itself, does not appear to exclude flame from the list of living

phenomena. So the definition fails to exclude an easy example of non-life.

It is worth pointing out here that the distinction between "easy" and "difficult" examples

that we made earlier is not as clear-cut as we pretended. The easy examples represent thecommon usage of modern people in industrialized countries, provided that they have not

studied too much philosophy. (Philosophy tends to warp our usage of words.) Thus, in

everyday life we would not say that an automobile or a candle flame is alive. But it wasnot always so, and less modern people might find it natural to regard flame as a sort of

living "fire spirit" because of its dynamic behavior. The point is that flame really is a

little bitlife-life.


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Barrow and Tipler raise another objection that strikes me as less serious:

On the other hand, tardigardes are simple organisms that can be dehydrated into apowder, and which can be stored in this state for years. But if water is added, the

tardigardes resume their living functions. When in the anhydrous state the tardigardes do

not metabolize. Are they "dead" material during this period?

Well, according to Schrodingers definition, yes, they are dead; because Schrodinger

talks of life in terms of its ability to keep "doing something" and the dehydratedtardigardes cannot do anything. In fact, one of the things they cannot do is reproduce!

Thus, this dehydrated state is equally problematic for the reproductively-based

definitions.

I believe the normal biological term for this state is "dormancy." A dormant tardigardebears a certain obvious resemblance to a seed that has not yet been planted and has not

begun to germinate. Is a seed alive? Well, not yet. But it certainly has the potential to

come alive, once the right conditions occur in its surrounding environment. Is thetardigarde alive? Well, not at the moment. It was before and it can be again.

Yet one is reluctant to classify the seed or the dormant tardigarde with things like rocks

or spoons, which are equally inactive. What is the difference? It is one of potential. Arock or a spoon will never "wake up" and start living. For convenience sake, then, we can

speak of seeds and tardigardes as having life in a "potential" form.

There is a partial analogy here to the concept of potential energy. When you throw a

baseball straight up in the air, it begins with a lot of kinetic energyenergy of motion.But it gradually slows down as it rises, finally stops, and falls at gradually increasing

speeds. Except for the effects of friction, it would reattain its original speed beforereaching ground level. A physicist would say that as the ball was rising and slowingdown, its kinetic energy was gradually being converted intopotential energy; and when it

was falling and speeding up, thepotentialenergy was being reconverted into kinetic

energy. This makes sense to physicists because they can then speak of the law ofconservation of energy: the total amount of energy in a closed system never changes, it

just gets transformed in various ways.

Ive digressed here to make a minor point, which is simply that when we speak of

potential life, this concept is not meant to imply anything like conservation of life. Life isnot conserved. Killing something does not create new life elsewhere; it just reduces the

number of living things in the world. And seeds can rot or get eaten before they

germinate, so the potential for life can also be destroyed. The concept of potential lifemerely allows us to distinguish between the possible futures of various types of currently

nonliving things.

But let us return to Schrodinger. In his book, he goes on to suggest just how it is that life

is able to resist entropy:


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How does the living organism avoid decay? The obvious answer is: By eating, drinking,

breathing, and (in the case of plants) assimilating. The technical term is metabolism. TheGreek word means change or exchange. Exchange of what? Originally the underlying

idea is, no doubt, exchange of material . . . That the exchange of material should be the

essential thing is absurd. Any atom of nitrogen, oxygen, sulphur, etc., is as good as any

other of its kind; what could be gained by exchanging them? For a while in the past ourcuriosity was silenced by being told that we feed upon energy . . . Needless to say, taken

literally, this is just as absurd. For an adult organism the energy content is as stationary as

the material content. Since, surely, any calorie is worth as much as any other calorie, one

cannot see how a mere exchange could help.

What then is that precious something contained in our food which keeps us from death?

That is easily answered . . . The device by which an organism maintains itself stationaryat a fairly high level of orderliness (= fairly low level of entropy) really consists in

continually sucking orderliness from its environment . . . In the case of higher animals we

know the kind of orderliness they feed upon well enough, viz. the extremely well-ordered

state of matter in more or less complicated organic compounds, which serve them asfoodstuffs. After utilizing it they return it in a very much degraded formnot entirely

degraded, however, for plants can still make use of it. (These, of course, have their most

powerful supply of "negative entropy" in the sunlight.)

The first time I read this, it seemed like a brilliant insight. However, problems arise when

you think about it more carefully. In his notes to the chapter, Schrodinger goes on to say:

But F. Simon has very pertinently pointed out to me that my simple thermodynamical

considerations cannot account for our having to feed on matter "in the extremely well

ordered state of more or less complicated organic compounds" rather than on charcoal or

diamond pulp . . . And so Simon is quite right in pointing out to me, as he did, thatactually the energy content of our food doesmatter; so my mocking at the menu cardsthat indicate it was out of place. Energy is needed to replace not only the mechanical

energy of our bodily exertions, but also the heat we continually give off to the

environment. And that we give off heat is not accidental, but essential. For this isprecisely the manner in which we dispose of the surplus entropy we continually produce

in our physical life process.

Schrodinger does not actually go far enough here to address the objections that F. Simon

raised. Schrodinger merely admits that, as well as being a source of negative entropy,

food is also important as a source of energy.

But consider the example of a steam locomotive. In a sense, it feeds on coal or some

other fuel. It burns the fuel, which enables it to perform work (moving forward). Like

living things, the locomotive degrades the coal and exhausts entropy (heat) to theenvironment. But can we really say that the fuel is its source of order? The order was

imposed on the locomotive in the factory where it was built, where the parts were

arranged in a certain fashion. The order is maintained by engineers and repairmen whoperform tasks such as cleaning, lubricating, and adjusting the mechanisms. The fuel


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contributes nothing to the orderliness of the machine, but simply provides it with motive

power.

Isnt it conceivable, therefore, that living things use food simply as a source of fuel,

rather than a source of order per se?

Well, that wouldnt be quite right. Food is not simply a source of fuel; it is also a source

of material which the body can use to build tissues and perform repairs. But if you look at

food from a standpoint of order, one of the first things you will notice is that the large-scale order in the food is destroyed before our bodies can assimilate it. First, we generally

dont eat something until after it is dead; then we chop it into pieces, cook it, and chew it

up into mush before swallowing.

What is more, the digestive process also breaks down many complex organic moleculesinto simpler forms before they are assimilated. Thus, all the proteins are broken down

into amino acids and only reassembled into proteins after they are digested. Further, the

body can use protein for more than one purpose: if more is available than is needed tobuild tissues, the body can burn the protein for energy instead. The food we eat does not

carry any mandate with it about how it should be used.

So the image of life as "sucking orderliness from its environment" is a little deceptive.Rather, the body sucks material from its environment. The body destroys most of the

order in this material, producing building blocks that are then absorbed and rearranged

into the bodys preexisting order.

Eating is so common that we tend to forget what an extraordinary process it is. We eatcorn flakes, and yet, we do not becomecorn flakes. The corn flakes are changed into us.

In [Richard] Feynmans splendid phrase, "Todays brains are yesterdays mashedpotatoes." I. S. Shlovskii and Carl Sagan,Intelligent Life in the Universe

However, it is true, as Schrodinger points out, that all life activities increase entropy, andthat the body must exhaust this entropy back into the environment (primarily by radiating

heat).

To summarize, what have we learned from Schrodinger?

Living things are systems with a characteristic order that persists over time.

Living things are active. (Even if an organism appears to be sitting still, processes aregoing on inside it.)

Living things are open systems that exchange material and energy with their

environment.

Living things increase the entropy in the environment around them.


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All these things appear to be true of life, but they do not constitute the definition we were

looking for. We know these factors are not a complete set of sufficient conditions,because they apply to phenomena such as candle flames that are not normally considered

alive. Besides that, we have not determined which of these criteria are minimal

independent criteria for life, and which are simply consequences that can be derived from

the more basic criteria.

Autopoiesis

Every five days you get a new stomach lining. You get a new liver every two months.

Your skin replaces itself every six weeks. Every year, ninety-eight percent of the atomsof your body are replaced. This nonstop chemical replacement, metabolism, is a sure sign

of life. This "machine" demands continual input of chemical energy and materials (food).

Chilean biologists Humberto Maturana and Francisco Varela see in metabolism theessence of something quite fundamental to life. They call it "autopoiesis." Coming from

Greek roots meaning self (auto) and making (poien, as in "poetry"), autopoiesis refers to

lifes continuous production of itself. Without autopoietic behavior, organic beings do not

self-maintainthey are not alive.

An autopoietic entity metabolizes continuously; it perpetuates itself through chemical

activity, the movement of molecules. Autopoiesis entails energy expenditure and the

making of messes. Autopoiesis, indeed, is detectable by that incessant life chemistry andenergy flow which is metabolism. Only cells, organisms made of cells, and biospheres

made of organisms are autopoietic and can metabolize. Lynn Margulis and Dorion

Sagan, What is Life?

The idea of metabolism was previously mentioned in Schrodingers definition. Thus, we

were already aware that life exchanges materials with the environment. However, the

idea of autopoiesis goes a little bit further than this. As Margulis and Sagan point out, it isnot just certain parts of the organism that participate in the exchange of materials with the

environment. Rather, every part of the body is involved in this exchange, and gradually

gets replaced over time.

Can we use autopoiesis as a single sufficient criterion for identifying life forms? Well, itcertainly fits the easy examples. You and I and all the animals and plants and bacteria are

autopoietic. Can we use this criterion to clarify the difficult examples? Margulis andSagan consider non-replicating examples such as mules:

Replication is not nearly as fundamental a characteristic of life as autopoiesis. Consider:the mule, offspring of a donkey and a horse, cannot "replicate." It is sterile, but it

metabolizes with as much vigor as either of its parents: autopoietic, it is alive. Closer to

home, humans who no longer, never could, or simply choose not to reproduce can not be


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relegated, by the strained tidiness of biological definition, to the realm of the nonliving.

They too are alive.

Margulis and Sagan go on to consider viruses:

In our view, viruses are not [alive]. They are not autopoietic. Too small to self-maintain,they do not metabolize. Viruses do nothing until they enter an autopoietic entity: a

bacterial cell, the cell of an animal, or of another life organism. Biological viruses

reproduce within their hosts in the same way that digital viruses reproduce withincomputers. Without an autopoietic organic being, a biological virus is a mere mixture of

chemicals; without a computer, the digital virus is a mere program.

Smaller than cells, viruses lack sufficient genes and proteins to maintain themselves. The

smallest cells, those of the tiniest bacteria (about one ten-millionth of a meter indiameter) are the minimal autopoietic units known today. Like language, naked DNA

molecules, or computer programs, viruses mutate and evolve; but, by themselves, they

are at best chemical zombies. The cell is the smallest unit of life.

Now consider the case of the candle flame. A flame is composed of gases that are

continuously in motion. New material enters through the bottom of the flame, and waste

products such as smoke and carbon dioxide exit through the top. All the molecules in theflame are regularly replaced, yet the flame itself persists. Is the candle flame an

autopoietic system? Seemingly it is. Thus it would seem that things can be autopoietic

without being alive.

It also seems conceivable that something couldbe alive without being autopoietic, at leastnot in the full sense of the constant and gradual replacement of molecules and cells.

Suppose that we succeed in creating artificial and mechanical life forms in the future.Such beings might require only the occasional replacement of major modules or

"organs." However, we are obviously indulging in speculation at this point.

Speculation alone should not perturb our definition greatly, but the case of the candle

flame does suggest that life has some additional essential feature that goes beyond

autopoiesis.

Responsiveness

Why dont we consider a candle flame to be living? Well, suppose we compare it withsomething that is living and see what the differences are. Let us compare the candle flame

with, say, a mouse.

Both the candle flame and the mouse need fuel. However, when the candle runs out of

wax, it is doomed. When the mouse runs out of food, it just starts exploring until it finds

some.


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You could argue that flames in general can spread to and engulf new fuel, such as when a

spark starts a forest fire. But there are limitations to this spreading, such as that fireoutdoors will tend to spread downwind. It may be that better sources of fuel are lying

upwind, and so never get used. A mouse, by contrast, can move upwind, or uphill. It can

see or smell food at a distance, or it can just keep searching around until it finds

something.

Note that this behavior is possible because the mouse doesnt have to feed at every

moment, as the flame does. It can store up energy inside and use it to survive until it finds

more food.

Now, besides the risk of running out of fuel, there are other hazards that can affect the

candle flame and the mouse. I can walk up to the candle flame and blow it out. But if I

decide to kill the mouse, I will find it a little more difficult. Most likely, it will see me

coming, and run for cover.

So the mouse is much better than the flame both at finding new fuel and evading dangers.Both of these factors involve the relationship between the mouse and its environment.

This type of observation must have inspired Herbert Spencers definition of life in

Principles of Biologyas "the continuous adjustment of internal relations to external

relations."

Barrow and Tipler quote this definition and disapprove of it, on the grounds that "such

definitions possess rather extreme ambiguities." Taken by itself, Spencers phrase is

certainly insufficient, because it doesnt make it clear what kind of adjustment is takingplace. Does he mean that the parts of the body actually try to mimic the arrangement of

items in the outside environment? It doesnt seem like there would be much point in such

a behavior. Still, there is something promising about Spencers definition, because itfocuses on the relationship between an entity and its environment; and, as we have seen,

this is an area where the mouse and the candle flame differ greatly.

In the bookDoubt and Certainty in Science, the British biologist J. Z. Young comparesliving things to rivers, because they have a pattern that persists even though new matter is

constantly flowing throw them. Young then goes on to say

But the [living] organization is vastly more complicated than that of any river. It is kept

in certain channels by the environment, acting in a sense as do the banks. If a streamstops, the banks remain, and therefore a river that has dried up may form again the same

patterns. But the living patterns are so complicated that they are kept intact onlyby their

continued activity. If they stop they are never restarted. The living patterns havedeveloped a wonderful permanence none the less. They have the characteristic that every

time there is any change in the banks the swirls make a compensating change and thus

keep intact.

Young abandons the metaphor at this point because it is difficult to go any further with it.

But he has already added something that goes beyond Spencers statement. The key


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phrase is "the swirls make a compensating change and thus keep intact." Of course, when

you think about it, this is a fairly obvious observation. The mouse does not run awayfrom food, nor does it run toward enemies. Staying alive is a serious business; it doesnt

happen by accident.

All this suggests a new definition of life, which I shall formulate as follows:

Living things are systems that tend to respond to changes in their environment in such a

way as to promote their own continuation.

When I say "new," of course, I mean only "new to me." Such a simple and obvious

definition has probably been proposed by someone before. Yet in my reading on the

subject, I have never run across it.

Of course, one can imagine some objections to this definition. Someone will pop up andsay, "What about the tardigardes when they have been dehydrated? They are not

responsive to anything." This is true, but as I have argued previously, they are also notliving in anything but a potential sense.

Of more serious concern are the cases where life acts against its own continuance. The

simplest example is that of the man who commits suicide. Now, it has been said that a

failed suicide attempt is actually a "call for help," and this may be so. But many of thesuicidal adopt methods that are reliable, and which they must have known in advance

would really kill them. So the objection here is, that the person who commits suicide was

indeed alive, but they were not acting in such a way as to stay alive. This seems to be a

counterexample to our definition.

Yet many or most cases of suicide result from mental illness. Such cases are comparableto any case where something has become broken and is no longer able to function

according to its original nature. You dont define the function of an automobile on the

basis of the way it behaves when it has a thrown rod or a broken head gasket.

Other cases of suicide may be quite rational, most notably among elderly people withpainful and lingering terminal diseases. But in that case you see that something is still

"broken." In this case, the mind is sound but the body is not functioning properly.

Then, of course, there are people who take needless risks, such as smoking cigarettes ordriving without seatbelts. These actions do not promote survival, but the risks are long-

term and difficult to comprehend vividly. Even people who smoke are not stupid enoughto walk in front of a speeding train. For all such people, we can conclude that the balanceof their actions tends to promote survival. But we can still find in these examples an

important addendum for our definition:

Living things are not perfect. Through internal breakdowns, mistakes, insufficient skill or

strength, or sheer bad luck, all living processes fail eventually and die.


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There is another important lesson to be drawn. Although the balance of life activities are

such as to promote survival, it does not follow that survival is the goal of life activities. Inother words, animals eat because they are hungry, not because they want to survive. But

the instinct to eat when hungry is one of many instincts that tend, on balance, to promote

survival.

The distinction may not seem important when we are talking about animals, but amonghumans it is crucial. We are able to understand the long-term impact that many of our

actions will have on our survival. Yet our ability to understand such things is a relatively

recent acquisition, and tends to get bypassed by more primitive instincts. Thus forexample, we tend to eat more and exercise less than is good for our health. Presumably in

ancient times food supply was irregular and incitements to physical activity were

plentiful, so our instincts were suitable for those times. Our intellectual understanding of

this fact does not always provide sufficient motivation to change our behavior today.

Thus, in general human beings are capable of doing things which we know do not

promote our chances for survival. Yet the balance of our activities must be such as topromote survival, or in fact we would not last very long.

Of course, many of our activities have no obvious bearing on survival at all. For instance,we come home from work and we decide to watch Star Trek or Seinfeld, or read a book

or do a crossword puzzle. These activities do not fulfill any immediate physical need. But

note that we pursue these activities within certain boundaries. We do not, as a rule, amuse

ourselves by running in front of cars on the freeway, or by hitting ourselves on the headwith a hammer. And our leisure activities presumably are driven by instincts that have

some general survival value, such as the urge to learn, to solve problems, or to form

social relationships.

So our proposed definition of life is consistent with a variety of observations about living

things. But seemingly it must collapse under the weight of a significant counterexample,

which is the fact that in nature, many things willingly sacrifice their own lives for the

sake of others. Let us go on to explore this theme in a little more depth.

Continuance Through Others

Let us begin by listing some of the situations in which living things have been known to

sacrifice their lives for others:

Mothers defend their children against attackers or potential attackers. For instance, a bird

will sometimes harass cats to keep them away from her nest.

People often attempt to rescue even total strangers from burning cars or other dangerous

situations.


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Honey bees fight intruders, such as bears, that threaten their hive. Whenever a bee stings

a bear, the bee dies from losing its stinger.

Soldiers or terrorists will sometimes agree to "suicide missions." An example is the

Japanese fighter pilots who would attempt to crash their planes into American naval ships

in WWII. Similarly, terrorists will sometimes hide a bomb under their clothing, go into acrowded public place, and detonate themselves.

These examples fall into two classes. In the first two, the living thing does somethingrisky, but in the second two, the living thing does something certain to lead to death. The

first and less extreme class is a lot more common than the second.

Let us examine the case of mothers defending their children first. What is the relationship

of mothers and their children? Aristotle has this to say:

For this is the most natural of the functions of such living creatures as are complete and

not mutilated and do not have spontaneous generation, namely to make another thing likethemselves, an animal an animal, a plant a plant, so that in the way that they can theymay partake in the eternal and the divine. For all creatures desire this and for the sake of

this do whatever they do in accordance with their nature . . . Now the living creature

cannot have a share in the eternal and the divine by continuity, since none of the mortalthings admits of persistence as numerically one and the same, but in the way that each

creature can participate in this, in that way it does have a share in it, some more some

less, and persists not as itself but as something like itself, not numerically one, but one in

species. De AnimaII.4, trans. Hugh Lawson-Tancred

In other words, because the individual cannot survive forever, it creates offspring similar

to itself so as to survive through them. Due to the vagaries of sexual reproduction, theoffspring resemble each parent only in certain respects. Thus, survival through oneschildren is only a partial and limited sort of survival. Yet all individuals die, so this

partial survival through children is the best backup plan available.

Today, Aristotles explanation of reproduction may seem a bit too purposive. He creditsall living things with an abstract desire to "partake in the eternal and the divine." And he

makes it sound as if living things all understand their own mortality and thus deliberately

choose to reproduce because of this. I. S. Shlovskii and Carl Sagan provide a more

modern spin to the importance of reproduction:

Can it be that reproduction is in some sense the "point" of biological activity? We canimagine an organism which carries out metabolism and all the other functions ordinarily

ascribed to living systems in elementary biology textbooks; which has very efficient

repair mechanisms, so that it easily survives the vicissitudes of its environment; andwhich has no reproductive organs and never reproduces. We can imagine such an

organism, but we can never find one. Why not? Because there is no way for such an

organism to arise. The only mechanism which we know for the production of biological

complexity is evolution by natural selection, the differential survival of organisms which,


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by chance, are best adapted to their environments. But natural selection can occur only if

the well-adapted organisms reproduce themselves. Thus, the development of complexityin living systems is intimately connected with self-replication. Intelligent Life in the

Universe

Evolution can provide organisms with reproductive instincts even if the organismsthemselves do not understand that they will die, and that the partial continuation through

offspring is their only long term chance for survival.

It makes perfect sense to regard reproduction as one application of a more basic

characteristic of life, which is its tendency toward self-preservation. By looking atreproduction in this way, we avoid the problems with the reproductively-based

definitions of life that we examined earlier.

Let us return to the example of the mother risking her life to protect her children. We can

now see that the mothers actions follow naturally from our definition of life. By

preserving her children, she is in some sense preserving herself as well.

Now consider the honey bee, which gives its life by stinging a bear that is attacking the

hive. The case is different here, for a worker bee can never have any offspring. Only the

queen and a small number of drones play any role in reproduction. In this organization,the workers only chance of long-term continuance is to protect the queens and drones. It

also needs to protect its fellow workers so that they, in turn, can help protect the queens

and drones; and it needs to protect the supply of honey in the hive that must be there to

enable them all to survive the winter. So in a general way the bee must defend the wholehive to ensure its own continuance, and thus it makes sense for it to give its life by

stinging the bear.

The examples of suicide missions and rescuing strangers are far less straightforward. Thesociobiologists, such as Edmund O. Wilson, have proposed that altruism can be a

genetically motivated trait, even in humans. However, we can also see that social forces

can and do encourage altruistic behavior in all of us. Whether the causes are genetic orcultural, the underlying logic is much the same. Human beings are generally social

creatures and our individual survival depends on our being part of a viable social group.

Actions that benefit other members of the group can thus have an indirect future benefit

either for the actor, or for relatives or descendants of the actor, or at the very least formembers of the same species. As Aristotle says, a living thing that gives its life for others

of its kind "persists not as itself but as something like itself, not numerically one, but one

in species."

Homeostasis

Following J. Z. Young, we have stated that living things are organizations or systems. In

other words, although the individual constituents are gradually replaced, their overall


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arrangement stays roughly the same or changes only gradually. This maintenance of a

constant state is referred to as homeostasis.

This is an interesting point because it causes us to look at life from the inside. During our

previous comparison of candle flames and mice, we focused on the externals of their

behavior, on their relationship with the environment. And this was appropriate to adegree. If you tried to define life strictly in terms of the relationships of its internal parts,you would be missing something essential. For none of the internal processes would be

possible unless the organism succeeded in obtaining food, evading predators, and so on.

Yet the threats to an organisms existence do not all come from the outside, and even theones that start on the outside do not always stay there. Disease organisms, whether

viruses or bacteria, often invade the system. In higher organisms, the immune system

exists to battle such invaders. The immune system also attacks the bodys own cells if

they become cancerous.

Aside from handling such outright threats, the body must coordinate a lot of complexinternal processes during the course of its day to day functioning. This is true even of

single-celled life forms. Hans Kupper points out that

Even in a simple bacterial cell there are estimated to be around a million functionalmolecules of two to three thousand different kinds. Each of these molecules carries out a

particular, specialized task, which in general is indispensable for the maintenance of the

functions of life.

Coordinating these functions involves the use of feedback. In other words, the internalprocesses are regulated according to need. A thermostat is a simple example of a

feedback mechanism: by turning the heater on or off, it affects the temperature, and thechanging temperature in turn affects the thermostat. The thermostat reacts in such a wayas to minimize the temperature deviations, and for this reason is said to be using negative

feedback. Negative feedback is a key element by which the organism coordinates its

internal systems and promotes homeostasis. Fritjof Capra gives a nice introduction to

feedback in his book The Web of Life.

Do we need to incorporate the concept of feedback into our definition of life? Well, it is

important to recognize that feedback per se does not necessarily promote life. Positive

feedback can increase the fluctuation in a system and cause it to self-destruct. Evennegative feedback can be inappropriate, if it preserves the stability of some subsystem at

the expense of the system as a whole. The central point seems to be that the ensemble of

inner processes tends toward homeostasis, the maintenance of the overall pattern of the

system.

Remember that we are searching for the minimal criteria to identify life. From this point

of view, it is sufficient to amend our definition by adding one simple clause:


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Living things are systems that tend to respond to changes in their environment, and inside

themselves,in such a way as to promote their own continuation.

We shall now go on to examine some of the concepts used in this definition in a bit more

detail.

Patterns of Complexity

Biology, like physics, has ceased to be materialist. Its basic unit is a non-material entity,

namely an organization. J. Z. Young,Doubt and Certainty in Science

Youngs statement follows from the fact of metabolism, the constant and gradual

replacement of every molecule in your body. Since the material flows in and out of theorganism, the organism cannot be considered to be a certain blob of matter, but is instead

a characteristic pattern in which matter is organized.

It happens that the nature of pattern and complexity is a topic that has recently received a

good deal of study. Following are some of the key results for the study of life.

Algorithmic Complexity

Starting in the 1960s, researchers including Gregory Chaitin, R. J. Solomonoff, and A.N. Kolmogorov created the branch of mathematics known as algorithmic information

theory (AIT). This theory defines patterns in terms of computation and computability. (A

number of articles on AIT can be found on Gregory Chaitins homepage at

http://www.cs.auckland.ac.nz/CDMTCS/chaitin/.)

For simplicitys sake, you can discuss all the issues relating to patterns in terms of binary

strings, that is, strings of zeroes and ones. This is the way that all information is stored in

computers. Even if something is three-dimensional, multicolored, and so on, there isalways a way of expressing that collection of properties as a long binary string. We will

refer to this as the representation stringfor that physical object.

Of course, we are not able to describe any physical system in complete detail. For

example, at the level of subatomic particles we run into quantum measurement problems.

In practice, when you create the representation string, you must choose some particular

level of detail and accuracy. This procedure is known as coarse graining.

Within AIT, apattern is defined simply as a program for generating a particular binary

string. A program is a collection of steps that can be executed on a computer.


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Note that every program on a computer is itself stored as a binary string. Within AIT, the

complexityof any given string is defined simply as the length of the shortest program thatwill generate that string. This program is referred to as the minimal programfor that

string. (The literature seems to imply that there could be more than one minimal

programprograms that are different, but of equal length.)

Actually, the length of the minimal program will vary to some extent, depending on thecomputer and the programming language that are used. Thus, when comparing the

complexity of different strings, it helps to define their complexity in terms of the same

computer system. (For very long strings, the differences between computer systems

become less and less significant.)

A particular string is said to be randomif the minimal program for generating that string

is about as long, or longer, than the string itself. Thus, randomness is also referred to as

noncompressibility; the inability to recreate something from anything shorter than itself.

Actually, the concept of algorithmic complexity does not correspond very well to theconcept of complexity in everyday life. You can see this because algorithmic complexity

is maximal for completely random strings. In everyday life, we draw a distinction

between randomness and complexity. Complexity does involve some kind of order,

though the order is not of a simple kind.

Logical Depth

The scientist Charles Bennett, of IBM, has suggested the concept of logical depth, as a

measure that more closely resembles our everyday idea of complexity. The logical depthof a string is defined as the number of runtime steps it takes for the minimal program tocreate that string. (This number can differ from the length of a program because a

program can include loops or subroutines that are executed repeatedly, and branches that

selectively skip statements.)

Rudy Rucker, in his bookMind Tools, says the following about logical depth:

Bennett and Chaitin, who are colleagues, speak of the two extremes of complexity ascrystaland gas. The atoms of a crystal have the property that they are very obviously

arranged according to a simple rule. They are like soldiers on a parade ground. The atoms

of a gas have the property that they are totally disordered, and are not arranged accordingto any rule much shorter than an actual listing of the atoms positions. Patterns that wefind interesting things such as living organisms and manmade artifacts lie midway

between the extremes of crystal and gas. One of the reasons Bennett invented the notion

of logical depth is that he wanted depth(gas) and depth(crystal) to be small, but

depth(organism) to be high.

And


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Bennett . . . argues that it may be appropriate to characterize living organisms as physical

structures that code up as bit strings with depths much larger than their lengths.

Now, logical depth couldbe a property of organisms. But Im not sure how you would go

about proving it. And it seems clear that logical depth is a property of at least some things

that are not organisms. Consider the Mandelbrot set, a favorite of computer hobbyistswho like to play with fractal mathematics. The Mandlebrot set is based on a simplefunction that, when applied repeatedly, generates some particular value for each point in

a two-dimensional plane. In pictures of the Mandlebrot set, different colors are assigned

to different ranges of values. Now imagine a physical object: a large and very beautifulposter representing some portion of the Mandlebrot set. The poster coloring can be

described by a very simple program that repeats a function repeatedly for each point on

the poster. In other words, the poster has great logical depth. But no one would mistake it

for a living thing.

Perhaps this seems too abstract, because a poster is flat and were just describing its

coloring. The Mandlebrot set can be represented in a three-dimensional way, so that itlooks something like a relief map with mountains and valleys. The height of each pointconveys the same information as the colors in the poster. So then you have a three-

dimensional object with tremendous logical depth. But it doesnt perform any of the

functions we have associated with life: it doesnt metabolize, it doesnt respond to theenvironment. It would not be classified as alive by any of the definitions we have

considered, not even the most nave ones.

Effective Complexity

In his book The Quark and the Jaguar, the physicist Murray Gell-Mann discusses another

type of complexity that he calls effective complexity. This concept, like the concept of

logical depth, is an attempt to supplement AIT with a concept closer to our intuitive sense

of what is complex.

Gell-Mann argues that in real life, we instinctively abstract out of our experience the

elements that are orderly. We use these orderly elements to develop and test mental

schema, which are models of selected elements of our experience. Gell-Mann defines theeffective complexityof a phenomenon as the length of the schema that describes the

regularities in that phenomenon.

For example, consider the following binary string:

1011101011001001011. . .

In this string, every third digit is "1":

1. . 1. . 1. . 1. . 1. . 1. . 1. . .


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The remaining digits have no obvious order:

. 01. 10. 01. 00. 00. 01. . . .

A schema for this string could describe only the regular component: the recurrence of 1 at

every third digit. As this is a very simple schema, the effective complexity of this string isquite low. The algorithmic complexity is much higher because the minimal program hasto be able to create the complete string, rather than just the portions that have obvious

regularities.

Ranges of Fluctuation

Let us step back and ask ourselves how these AIT concepts apply to living things. If a

living thing is an organization, then it must be possible to construct either a pattern that

gives create a complete description of the organism, or a schema that gives a completedescription of the regular elements in the organism. Further, that program or schema must

be shorter than the representation string for the organism.

Does such a program or schema exist? At first glance, it might seem that the DNA of an

organism plays such a role. It certainly is reasonable to describe DNA as a program,which is executed by the cellular "computer" consisting of RNA and proteins. It includes

a plan for building the organism from scratch (morphogenesis) and instructions for the

behavior of each type of cell.

However, the organism is shaped and modified by environmental influences, and the

DNA does not record these. For example, as we learn and experience the world, our brainstructure changes through the growth of dendrites and so on. Similarly, the formation ofthe body in childhood can be affected by the availability of various nutrients. More

disturbing, perhaps, is the following observation:

An average human is normally host to billions of symbiotic organisms belonging to

perhaps a thousand different species . . . His phenotype is not determined by his humangenes alone but also by the genes of all the symbionts he happens to be infected with. The

symbiont species an individual carries usually have a very varied provenance, with only a

few being likely to have come from his parents. Juan Delius, "The Nature of Culture,"in M. S. Dawkins, T. R. Halliday, and R. Dawkins, eds., The Tinbergen Legacy; quoted

in Daniel C. Dennett,Darwins Dangerous Idea

Further, the body is changing all the time. On the microscopic level, the actual

constituents of the body are constantly changing through metabolism. On the

macroscopic level, the overall shape of the body changes every time we perform avoluntary or involuntary movement (walking, breathing, etc.). What is more significant,

the changes that the organism goes through are often in response to environmental

conditions that cannot be predicted in advance.


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Thus, if you regard DNA as apattern of the organism, you must be using a representation

string that was developed at an extreme level of coarse graining. It would actually be alevel much coarser than our everyday perception of living things, which includes an

awareness of features of organisms, such as characteristic behaviors, that are strongly

shaped by the environment.

To regard DNA as a schemafor living things is equally unsatisfactory. A schemadescribes the regularities in a phenomenon, but as we have seen, an organism can acquire

regularities through experience. This may seem paradoxical, but an acquired trait needs to

be considered a regularity because it persists and affects all future behavior, as well as

affecting the acquiring of new traits.

An additional problem arises because schemata describe regularities, yet organisms do

not seem to have any characteristics that are perfectly regular. Certainly the relativepositions of the limbs change all the time. Aside from that, a property such as

temperature fluctuates around an average. Our height also varies on a daily basis, due to

compression of the soft parts of the spine during the day. The ears and nose continuegradually growing during adulthood. Bones seem permanent, but the calcium in bonescan be temporarily shifted to other parts of the body, or the bones can gradually waste

away, resulting in osteoporosis. Our eyes may be bloodshot one day and white the next.

Our DNA itself frequently develops errors, and various error-correction mechanisms

exist within the cell to compensate for this fact.

Paul Weiss formalized the idea of a system is in the following way:

Weiss now denotes the complex Sas a systemif the fluctuations in the properties of the

whole complex are significantly smaller than the sum of the fluctuations for the

subsystems. Hans Kupper,Information and the Origin of Life

This gives us an interesting way of thinking about living systems. All the apparent

regularities in an organism appear really to be fluctuations around average values. In

addition to this, the average values themselves gradually change over time as the

organism passes from infancy into adulthood and decays into old age.

To describe the order in an organism, then, you would need to give a list of properties

and explain the characteristic ways in which each property varies: the average value, and

the range, rate, and cause of variation, among other things.

Such a description could be considered a schema of a sort, but it has a certain mushyquality to it. As a result, to recognize a particular organism, you must make use of

mechanisms with mushy or gradual constraints. The disciplines of fuzzy logic and neural

network theory have developed examples of such mechanisms.

Behavior


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We have been considering what it means to say that living things have some

characteristic order, or pattern of organization. The most useful concept we have foundfor describing that order is Paul Weisss notion of a system as a collection of properties

with fluctuating values.

But this is only one aspect of life. We have next to explore what it means when we saythat a living thing responds to changes in its environment, in such a way as to promote itsown continuation. We can begin by asking whether this property, behavior, is really a

universal property of life. S. E. Luria, Stephen Jay Gould, and Sam Singer have this to

say in their bookA View of Life:

In the broadest sense of the term, behavior consists of everything an organism does as it

goes about the business of survival and reproduction, including reflex activities that help

maintain the constancy of its internal environment. It is therefore not surprising thatanimals, especially vertebrates, with their elaborate brains and active life-styles, engage

in the most complex behavior of many organisms. But behavior is by no means limited to

animals with complex brains, or even to animals.

Unicellular organisms, both prokaryotic and eukaryotic, are capable of several kinds of

behavioral responses to specific stimuli. For example, some species of bacteria thatcontain particles of iron oxides in their cytoplasm preferentially orient themselves in

certain directions when places in weak magnetic fields. Whether or not this simple

behavior has any adaptive significance remains to be discovered . . .E. coliand some

other bacteria react to certain chemicals in their environments by changing the directionin which their flagella rotate. This response occurs because these bacteria have some kid

of processing mechanism in their cell membranes that causes them to swim towards

nourishing chemicals and away from noxious ones.

Protists, with their elaborate unicellular body plans, have a behavioral repertoire that is

correspondingly more complex than that of prokaryotes. An amoeba, for example, can

detect the presence of edible cells and particles in its immediate vicinity and, as long asthe intended meal does not move too far out of range, will relentlessly pursue it by

extending its pseudopods. Amoebas also show specific behavioral responses when

exposed to light. Bright light causes an advancing pseudopod to be withdrawn. And whenan entire amoeba is suddenly exposed to bright light, it draws in all of its pseudopodia

and vigorously contracts to form a spherical blob of protoplasm.

As a group, plants are among the least motile organisms and their behavior is

correspondingly less complex than that of creates that actively move through theenvironment in search of food, shelter, and mates. Nevertheless, many plants do engage

in simple, predictable behaviors. As discussed in the previous chapters, most plants react

to light by bending toward it, and roots grow toward the pull of the earths gravitationalfield, while stems preferentially grow away from it. Some plants move their leaves in

response to touch. This is true of the Venus fly trap, which rapidly closes a pair of hinged

leaves to trap insects, and of the "sensitive plant" (Mimosa pudica), which upon being

touched, heated, or stimulated by an electrode rapidly folds up its leaves and leaflets.


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But what is behavior, in general? What do all these examples of specific behaviors have

in common? Let us for the moment be a bit less general than mssrs. Luria, Gould, andSinger, and restrict ourselves to the interactions between an organism and its external

environment. There seem to be two factors in behavior: input and output. The organism

is, as it were, struck by something from outside of itself, and as a result of this, the

organism moves.

But a billiard ball also is struck by something outside of itself, and also moves as a result.

Yet we do not say that it respondsto its environment. The motions of living things have

the following features:

Motion often involves the expenditure of more energy than was contained in the input.

Aristotle intimates something similar inDe Motu Animalium, Chapter 7: "It is not

difficult to see that a small change occurring in an origin sets up great and numerousdifferences at a distancejust as, if the rudder shifts a hairs breadth, the shift in the

prow is considerable."

For example, suppose I see a tiger and flee. In this case, a small amount of light struck

my retina. The energy supplied was not enough to move my body significantly. Yet my

body did move and it took a great deal of energy to make it move. The necessary energy

was supplied from within, by converting stored chemical energy into kinetic energy.

Similarly, a plant struck by light from a certain direction, will tend to expend its own

resources to send out new growth in that direction.

Motion does not necessarily continue in the same direction as the input, nor is itnecessarily modified in the same direction is the input. In other words, you cannot simply

calculate the subsequent movement of the organism by adding vectors that describe themomentum of the input to vectors that describe the current momentum of the organism.

Actually, the laws of physics still apply to organisms. If you shoot a man out of a cannon,you can predict his path in much the same way as you do that of a cannon ball. But the

majority of our movements are not the result of such massively energetic inputs, and sothe initial effect of the impact is overwhelmed by the bodys internally generated

response.

Motion may result from input after some considerable delay. For example, if a man

pinches a woman, she may slap him a moment later, or she may file a lawsuit against him

a year later.

Motion is made possible by internal variations in the organism that tend to remain within

typical ranges of fluctuation. For example, muscles tighten or relax to cause the

movements of limbs.

Different species exposed to similar input may move in different ways.


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Within a species, different individuals exposed to similar input may move in different

ways.

The same individual, when exposed to similar input on different occasions, may move in

different ways.

An organism, on encountering an obstacle to motion, may go around the obstacle and

continue motion in the original direction.

To summarize, the relationship between the input and the response is an indirect one. Thepoints listed above are various types of indirection. Such examples could probably be

multiplied.

The causal relationship between input and output is a complex one because of things that

happen inside the organism. Internal processes mediate between the input and theresponse, and as a general term I shall refer to such processes as mediation. Mediation

can include, but is not limited to, the phenomenon of intelligence. For instance, reflexreactions are also the result of internal mediation, though it is of a primitive sort.

Indeed, it could be argued that inputs are not the causes of our actions at all, since as we

have seen, no particular response follows necessarily from any particular input. Further,

you can be sitting thinking, undisturbed by the conditions around you, and then suddenlyreach a conclusion that causes you to take some action. Aristotle again, inDe Motu

Animalium:

For the animal moves and progresses in virtue of desire or choice, when some alteration

has taken place in accordance with sense-perception orphantasia. . .

This, then, is the way that animals are impelled to move and act: the proximate reason formovement is desire, and this comes to be either through sense-perception or through

phantasiaand thought . . .

For sense-perceptions are at once a kind of alteration andphantasiaand thinking have the

power of the actual things. For it turns out that the form conceived of the [warm or coldor] pleasant or fearful is like the actual thing itself. That is why we shudder and are

frightened just thinking of something.

The termphantasiain the passage is apparently an untranslated Greek word for

imagination.

Now, all this is true but it does not follow that our actions are uncaused, or even uncausedby input from the outside. For even in the case where we are sitting thinking, and

suddenly decide to do something, the mind that is doing the thinking has been

progressively modified by many inputs over the years. What does follow is that theseinputs are not the completecauses of action. You put a key in a lock to open the door, but

the key could have no effect unless the lock were shaped to receive it. The door opening


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is caused not only by the key turning, but also by all the forces that shaped the lock that

receives it. Our responses are caused not just by some stimulus, but also by all the factors

that have shaped us, both genetic and environmental.

Men think themselves free inasmuch as they are conscious of their volitions and desires,

and never even dream, in their ignorance, of the causes which have disposed them so towish and desire. Benedict de Spinoza, The Ethics, Appendix to Part I, trans. R. H. M.

Elwes

While the considerations in this section are helpful secondary criteria for identifying life,

they are not sufficient, either individually or in combination, to finally show thatsomething is alive. For we might be able to create some mechanism that responds to

inputs in some sophisticated and indirect way. But if its responses are not such as to

promote survival, such as by avoiding threats and seeking out food, then it is not really

behaving in the manner characteristic of life.

A Matter of Degree

Why are there "difficult" examples to address in definitions of life? Take the example ofviruses. We have seen, so far, that Barrow and Tipler regard the virus as alive, whereas

Poundstone and Margulis and Sagan regard it as not alive. Luria, Gould, and Singer offer

the following conclusion in their bookA View of Life:

Are viruses alive? This question is more difficult to answer because it depends on adefinition of life. Suppose our definition includes the idea that living things are able to

reproduce. A dog is obviously alive and is made up of living cells, but a spayed dogcannot reproduce and its genetic information dies with it; yet is alive. We may, on theother hand, define life as the possession of specific genetic information capable of

functioning in living cells. Then the cells of the spayed dog are clearly alive, and so are

viruses, which can multiply in living cells. Viruses reproduce and evolve if they havesuitable host cells available. Are viruses any different from animals or plants, which also

require specific external conditions to propagate their species? To the biologist, a virus is

alive because it participates in the adventure of biological evolution.

The last sentence seems to imply a consensus, at least among biologists; but we have seenthat so eminent a biologist as Lynn Margulis regards viruses as non-living. Now consider

the following:

Nature proceeds little by little from things lifeless to animal life in such a way that it is

impossible to determine the exact line of demarcation. Aristotle, The History of

Animals, viii: 1. Cited in Lynn Margulis and Dorion Sagan, What is Life?


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Could it be that the property of life is something that can be possessed in varying

degrees? Actually, this notion follows fairly naturally from the definition of life that we

proposed earlier:

Living things are systems that tend to respond to changes in their environment, and inside

themselves, in such a way as to promote their own continuation.

It seems clear that different systems could display varying degrees of this tendency. But

do they actually? Well, we have seen that flames have a very limited ability to sustainthemselves. Viruses have the ability to reproduce, but little else. In cells you find this

reproductive ability as well as more advanced techniques for maintaining the integrity of

the individual, such as by storing energy from sunlight or swimming toward edible foods.

However, if you visualize a graph of beings that display various deg

the definition of life

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