logic professor d’ascoli hum 200 strayer university
TRANSCRIPT
LOGIC
Professor D’Ascoli
HUM 200
Strayer University
WHAT IS LOGICAL?
LOGIC
WHAT IS LOGIC?
Lewis Carroll, Through the
Looking Glass: “Contrariwise,
continued Tweedledee, \if it
was so, it might be; and if it
were so, it would be; but as it
isn’t, it ain’t. That’s logic."
THE TEXT B OOK TYPE DEFINITION
Logic, from the Greek
λογικός (logikos) is the
study of reasoning. Logic is
used in most intellectual
activity, but is studied
primarily in the disciplines
of philosophy, mathematics,
and computer science.
SOME DEFINITIONS OF LOGIC
the branch of philosophy that analyzes inference
reasoned and reasonable judgment; i.e. "it made a certain kind of
logic"
the principles that guide reasoning within a given field or
situation; i.e. "economic logic requires it"; "by the logic of war"
the system of operations performed by a computer that underlies
the machine's representation of logical operations
a system of reasoning
LOGIC IS MATHEMATICAL
LOGIC IS ARGUING IN A REASONABLE WAY
LOGIC – OUR TEXT
Logic is the study of reasoning: how it is
done correctly, how it goes wrong, and
how to distinguish between the two.
Reasoning involves constructing and
evaluating arguments.
PROPOSITIONS
Arguments are made up of propositions.
In an argument, we attempt to establish the truth of a proposition
on the basis of others.
Propositions are assertions that are either true or false.
A simple proposition makes only one assertion.
Compound propositions contain two or more simple propositions.
Compound propositions can be either disjunctive or hypothetical.
PROPOSITIONS
There are many propositions about whose truth we are uncertain.
Examples:
There is life on other planets.
There is a God.
These may be true or false, therefore their ‘truth value’ is
uncertain.
However, these, like all other propositions must be either true or
false.
PROPOSITIONS
Questions are not propositions as they assert no truth values (Do
you like this class?)
Commands and exclamations are also not propositions as they also
assert no truth values (ie, Come here. Watch Out!, etc)
Sentences can take many forms to assert the same thing, even
different languages can assert the same thing
This class is stupid
Esta clase es estúpida
Cette classe est stupide
PROPOSITIONS
Propositions and statements are not exactly the
same, but they are often used in logic in the same
sense.
Some logic texts even use the word statement
rather than proposition. We will use proposition
PROPOSITIONS
Some propositions (compound) contain more than
one proposition in the same sentence: China is the
most populous country in the world, it produces 85%
of the world’s goods and has a communist
government.
The above is also an example of conjunctive
propositions – though listed together , they could all
be listed separately and still be true.
PROPOSITIONS
However, there could also be compound
propositions that are disjunctive , where no one of
the components is asserted.
For example, “Circuit courts are useful, or they are
not useful.”
Although this example is clearly a true but one of
its components might be false.
PROPOSITIONS
There are also hypothetical (or conditional) propositions – these are
compound propositions that also do not assert that their components
are true but rather that the whole is true.
Example : “If God did not exist, it would be necessary to invent him.”
Again neither claim is asserted, rather it becomes an if then
hypothetical dilemma which even if both parts are wrong, may still be
a true proposition – because it is hypothetical
You can never successfully argue hypotheticals
Excercises pages 9-12
IS THIS AN ARGUMENT?
http://www.youtube.com/watch?v=teMlv3ripSM
YouTube - Monty Python - Argument Clinic
LOGICAL FALLACY?
http://www.youtube.com/watch?v=yp_l5ntikaU
YouTube - monty python-witch scene
Professional Logician monologue -
YouTube - Monty Python and the Holy Grail Soundtrack 3/7: Logician
http://www.youtube.com/watch?v=FZqs36C5sgM&feature=related
Good evening. The last scene was interesting from the point of view of a professional
logician because it contained a number of logical fallacies; that is, invalid propositional
constructions and syllogistic forms, of the type so often committed by my wife. "All wood
burns," states Sir Bedevere. "Therefore," he concludes, "all that burns is wood." This is, of
course, pure bullshit. Universal affirmatives can only be partially converted: all of Alma
Cogan is dead, but only some of the class of dead people are Alma Cogan. "Oh yes," one
would think.
However, my wife does not understand this necessary limitation of the conversion of a
proposition; consequently, she does not understand me. For how can a woman expect to
appreciate a professor of logic, if the simplest cloth-eared syllogism causes her to flounder.
For example, given the premise, "all fish live underwater" and "all mackerel are fish", my
wife will conclude, not that "all mackerel live underwater", but that "if she buys kippers it
will not rain", or that "trout live in trees", or even that "I do not love her any more." This
she calls "using her intuition". I call it "crap", and it gets me very *irritated* because it is
not logical.
"There will be no supper tonight," she will sometimes cry upon my return
home. "Why not?" I will ask. "Because I have been screwing the milkman all
day," she will say, quite oblivious of the howling error she has made. "But," I
will wearily point out, "even given that the activities of screwing the milkman
and getting supper are mutually exclusive, now that the screwing is over, surely
then, supper may, logically, be got." "You don't love me any more," she will now
often postulate. "If you did, you would give me one now and again, so that I
would not have to rely on that rancid Pakistani for my orgasms." "I will give you
one after you have got me my supper," I now usually scream, "but not before" --
as you understand, making her bang contingent on the arrival of my supper.
"God, you turn me on when you're angry, you ancient brute!" she now
mysteriously deduces, forcing her sweetly throbbing tongue down my throat.
"Fuck supper!" I now invariably conclude, throwing logic somewhat joyously to
the four winds, and so we thrash about on our milk-stained floor, transported by
animal passion, until we sink back, exhausted, onto the cartons of yoghurt.
I'm afraid I seem to have strayed somewhat from my original brief. But in a
nutshell:
Sex is more fun than logic -- one cannot prove this, but it "is" in the same
sense that Mount Everest "is", or that Alma Cogan "isn't".
Goodnight.
IS THIS AN ARGUMENT?
1. Ms. Malaprop left her house this morning.
2. Whenever she does this, it rains.
_____________
3. Therefore, the moon is made of blue cheese.
ARGUMENTS
Inference is the process that may tie together a cluster of
propositions, some are warranted (correct) others are not
An argument in logic does not refer to a disagreement
An argument refers strictly to any group of propositions of
which one of the propositions is claimed to follow from the
other propositions
For every possible inference there is a corresponding
argument
ARGUMENTS
Although sentences express propositions, a
sentence and a proposition are not identical.
The propositions that provide evidence or support
for the truth of some other proposition are called
premises.
The proposition for which evidence is provided is
called the conclusion.
ARGUMENTS
Sometimes premise and conclusion appear in separate sentences:
“No one was present when life first appeared on earth. Therefore any
statement about life’s origins should be considered as theory, not fact.”
Sometimes they appear in same sentence:
“Since it turns out that all humans are descended from a small
number of African ancestors in our recent evolutionary past, believing
in profound differences between the races is as ridiculous as believing
in a flat earth.”
ARGUMENTS
The order in which premises and conclusions can
appear are also varied. This does not matter in
determining validity or soundness of arguments.
ARGUMENTS
Arguments often contain conclusion and premise indicators
that allow one to identify them as arguments.
When indicators are lacking, the context of the passage
provide cues as to whether it is argumentative in nature. P
13 – 14 let’s discuss
Once an argument is identified, care must be taken to
identify premises which are not in a declarative form or
premises that are unstated. P 14 -15-16-17-18-19
CONCLUSION INDICATORS
Therefore - for these reasons
Hence - it follows that
So - I conclude that
Accordingly - which shows that
In consequence - which means that
Consequently - which entails that
Proves that - which implies that
As a result - which allows us to infer that
For this reason - which points to the conclusion that
Thus - we may infer
PREMISE INDICATORS
Since - as indicated by
Because - the reason is that
For - for the reason that
As - may be inferred from
Follows from - may be derived from
As shown by - may be deduced from
Inasmuch as - in view of the fact that
ARGUMENTS
Arguments must be distinguished from other forms
of expression involving sets of propositions, for
instance, expository passages and explanations.
An explanation is a group of statements that
purport to account for why something happened or
why something is the way that it is. P 19-20-21
excercises pages 21-26
DEDUCTIVE ARGUMENTS
Some arguments are deductive, and some inductive—and all arguments
are either one or the other.
Deductive arguments claim that if the premises are true, the conclusion
follows with absolute necessity. That is, it cannot be false.
In valid deductive arguments, if the premises are true, the conclusion
does, indeed, follow with absolute necessity.
An invalid deductive argument is one in which, if the premises are true,
the conclusion could be false.
A sound deductive argument is one that is valid and has all true
premises.
DEDUCTIVE ARGUMENTS
The relationship between true (or false)
propositions and valid (or invalid) arguments is
sometimes quite complex.
The only combination of premises and conclusion
whose truth-values guarantee the invalidity of the
argument is when the premises are true and the
conclusion false
INDUCTIVE ARGUMENTS
In inductive arguments, the conclusion is claimed
to follow only with high probability.
Inductive arguments are never valid or certain;
they can be better or worse, more or less probable,
but they can never be valid or invalid.
P 28-30
IS THIS ARGUMENT VALID?
1. If the moon is made of blue cheese, then pigs
fly.
2. The moon is made of blue cheese.
______________
3. Therefore, pigs fly.
WHAT WE AIM FOR
An argument is sound if and only if the
argument is valid and, in addition, all of its premises
are true.
VALIDITY AND TRUTH
Valid (validity) – refers to the relation between its
propositions only, if the conclusion follows with logical
necessity from the premises then an argument is said to be
valid
Validity can never refer to a single premise by itself
Truth – is the attribute of a proposition that asserts what
really is the case
Truth cannot apply to arguments
VALIDITY AND TRUTH
Truth and falsity are attributes of individual
propositions or statements; validity and invalidity are
attributes of arguments.
P 31-33 discuss samples
Excercises page 35
KEY TERMS
Proposition Argument Premise
Statement Conclusion Probability
Validity Induction Necessity
Soundness Deduction Simple proposition
Compound proposition Disjunctive proposition
Hypothetical proposition
Classical logic Modern symbolic logic Explanation
Explanation Inference Enthymemes
SOME QUESTIONS/ DISCUSSION
1. Why is logic relevant to everyday life? Why should one take a course in
logic?
2. We often rely on appeals to emotion in order to persuade people rather
than providing arguments. Give some examples of this from everyday
contexts. Is this problematic? Are there cases when appeals to emotion are
appropriate?
3. Give an example of a simple argument you have made recently. Which
statements are the premises? Which one is the conclusion?
4. What is the distinction between deductive and inductive arguments? Give
an example of each to make your explanation clear.
5. What is the difference between validity and soundness? Why is the
distinction relevant for us as students of logic?
HOMEWORK QUESTIONS
1. What is the difference between a premise and a
conclusion? Provide an example of an argument from a
newspaper or journal that highlights this distinction.
2. Why is reasoning considered to be both an art and a skill
and how does taking a course in logic help us to develop that
skill?
3. What is the difference between inductive and deductive
arguments? What are the ramifications of this difference?
HOMEWORK CONTINUED
4. A valid argument does not necessarily mean that the premises
and the conclusion are true. In some cases, a deductive argument
will be valid even when its premises and conclusion are false. If
validity doesn’t mean truth, why should a logician be concerned
with validity?
5. In everyday contexts, we are confronted with argument in a
variety of different spheres; political, religious, legal, medical,
and so on. Why is it important to be able to analyze and assess
these arguments?