from physical explicability to full-blooded materialism

Scots Philosophical AssociationUniversity of St. Andrews

From Physical Explicability to Full-Blooded MaterialismAuthor(s): Robert KirkSource: The Philosophical Quarterly, Vol. 29, No. 116 (Jul., 1979), pp. 229-237Published by: Oxford University Press on behalf of the Scots Philosophical Association and theUniversity of St. AndrewsStable URL: http://www.jstor.org/stable/2218819 .

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229

FROM PHYSICAL EXPLICABILITY TO FULL-BLOODED MATERIALISM

BY ROBERT KIRK

Many philosophers would agree that any particular physical phenomenon which is explicable at all has some physical explanation. But not all who agree with this statement would willingly label themselves materialists. Yet, as I shall try to show, acceptance of this doctrine of physical explicability cannot sensibly be combined with rejection of a strong variety of materialism, namely what I call the Implication Thesis. This is to the effect that mental and psychological facts are strictly implied by the set of all actually true physical sentences. The tendency of my argument is thus to force a choice between acceptance of full-blooded materialism and rejection of the thesis of physical explicability.

First we need reasonably clear statements of the two theses in question. We can start by defining a sentence or description (viz., an open sentence with one free variable) as physical if and only if it is couched in the vocabulary of today's physics, supplemented by the vocabularies of mathematics and logic, and equipped with a system of spatio-temporal co-ordinates. I shall assume that all distinctively mental or psychological words are excluded from this physical vocabulary. Sentences and descriptions will be counted as mental if and only if they include essential occurrences of mental or psychological words. (Fortunately the argument does not depend on precise definitions of these wordsl) Note that statements of physical laws and indeed of whole physical theories are included among the physical sentences.

Physical and mental phenomena can be defined in terms of physical and mental sentences. True, we cannot just say that a phenomenon is physical (or mental) if it has a physical (or mental) description, since physical phenomena can often be picked out in mental terms, by relating them to their causes or effects. However, we can give adequate definitions for present purposes if we confine ourselves to non-relational descriptions. A non- rekational description of x will be a description whose being true of x does not depend logically on the relations in which x stands to particular things external to it. A phenomenon (viz., for our purposes, an object, event, state, process, fact or property) will be counted as physical (or mental) if and only if it has some non-relational physical (or mental) description. This does not prevent the same phenomenon from being counted as both physical and mental; but it does ensure that not all phenomena are counted as mental.'

1A definition of 'mental event' suggested by Donald Davidson allows all events to be counted as mental ("Mental Events", in Experience and Theory, L. Foster and J. W. Swanson (edd.), London, 1970, 79-101). But that would defeat my present purposes.



230 ROBERT KIRK

I can now state the doctrine of Physical Explicability: Every particular physical phenomenon that can be causally explained at all has some causal explanation consisting wholly of physical sentences.

Note that this doctrine does not commit one to universal causal determinism or universal predictability. Nor does it necessarily preclude the possibility that schemes of explanation and description other than the physical-e.g., psychological schemes-should apply to some of the same phenomena as does the physical scheme.

Next, a definition of strict implication. A set of sentences S strictly implie a set of sentences T if and only if it is impossible for linguistic or conceptual reasons, broadly construed, that all the members of S should be true when some member of T is false. The vagueness of this characterization is unavoidable, I think, until more is known about meaning and truth. The idea is that the impossibility in question is somehow non-empirically deter- mined, and I shall turn a blind eye to the difficulties raised by this idea. Notice especially that strict implication in the intended sense need not pre- suppose that all words used in the implied set can be analysed in terms of the implying set. So if the set S strictly implies the set T in this sense it need not follow that if one knew the members of S one could "read off" the members of T. One illustration of these points must suffice. Consider these two sentences:

(1) A certain possible individual b is exactly like an individual a in all physically describable respects.

(2) a has a sense of humour. A verificationist might maintain that certain general conditions for meaning- fulness makes it impossible for (1) and (2) to be true if the following sentence is false:

(3) b has a sense of humour. Such considerations would be putative justification for thinking that (1) and (2) strictly imply (3) in the sense in question-even though there was no discussion of the specific meaning of 'x has a sense of humour'.

The neatest formulation of the Implication Thesis would have the set of all true physical sentences strictly imply all true sentences. Unfortunately this will not quite do, since conceivably materialism might be true even if certain odd sentences, such as 'Socrates does not have an immortal soul', were true but not strictly implied by the set of all true physical sentences. Instead, the supposedly implied sentences must be a subset of all true sentences. I shall call them the target sentences and characterize them as the physical sentences together with all those non-compound non-physical sentences whose falsity would obviously raise no problems for materialism, but whose truth might be thought to do so. Examples of target sentences: 'Socrates has a greenish after-image', 'Some people deceive themselves'. Examples of sentences that are not target sentences: 'Socrates does not



FROM PHYSICAL EXPLICABILITY 231

have an immortal soul', 'This stone is not thinking about Vienna'. There seems to be no point in trying to define the target sentences precisely.

The Implication Thesis can now be stated thus: The set of all actually true physical sentences strictly implies the set of all true target sentences.

It matters that the impossibility invoked in the Implication Thesis is not merely empirical or physical. Even epiphenomenalists, even adherents of the doctrine of Pre-established Harmony, could accept the Implication Thesis if the impossibility in question were only empirical. However, the fact that the impossibility involved is non-empirical does not automatically render the Implication Thesis non-contingent. For the thesis may be represented symbolically as follows, where 'Ixy' stands for '[the set] x strictly implies [the set] y', 'Tx' stands for 'x is a true target sentence', and 'Px' stands for 'x is a true physical sentence':

(3x)(3y)[Ixy & (z)(Tz {(zey) & (Pz = (zex))})]. That a statement of this form may be contingent can be seen by reinterpreting 'Tx' as 'x is a sentence written by me today' and 'Px' as 'x's first letter is 'P' '. For it is obviously contingent whether or not the set of those sentences I write today which start with a 'P' strictly implies the set of all the sentences I write today.

By now I hope it is reasonably clear what the Implication Thesis is about. If all true target sentences are str.ctly implied by the true physical sentences, then whatever is logically sufficient for the truth of the latter is also logically sufficient for the truth of the former. This is a strongly material istic thesis even though it does not require mental sentences to be translat- able in neutral or physical terms. In one sense it is a "two-language" variety of materialism, because according to it mental and other non-physical descriptions and explanations are just other ways of describing and explaining physical phenomena.

My aim in this paper, then, is to show that no one who accepts Physical Explicability can sensibly reject the strong form of materialism represented by the Implication Thesis (given that qualms about strict implication are to be stifled). "Sensibly": I do not claim that the former entails the latter, only that acceptance of the former and rejection of the latter would require either the denial of something we all know to be true, or else indulgence in gratuitous metaphysical extravagance.

II The crucial step will be to show that if the thesis of Physical Explic-

ability is true, so is the following: (A) If a particular mental phenomenon has some physical effect,

then some true mental sentence describing that phenomenon is strictly implied by the set of all actually true physical sentences.

The argument from Physical Explicability to (A) can be conveniently pre-



232 ROBERT KIRK

sented by means of an example. Let us take the particular mental phenomenon of Napoleon's headache and assume that

(1) Napoleon's headache [or his having a headache, his headachy state, or whatever-nothing hinges on the category of item involved] contributes to bringing about the physical events involved in Napoleon's frowning.

Assume too that-contrary to what I shall try to show- (2) No true mental sentence describing Napoleon's headache is strictly

implied by the set of all actually true physical sentences. Assume finally the thesis of Physical Explicability:

(3) Every particular physical phenomenon that can be causally explained at all has some causal explanation consisting wholly of physical sentences.

We shall see that these three assumptions are inconsistent. Given the definition of strict implication, (2) obviously implies that:

(4) The world could, logically, have been in all physically describable respects exactly as it actually is even if Napoleon had had no headache.

Of course the physical events involved in Napoleon's frowning are among the phenomena that would remain unchanged in the situation envisaged. Now since the set of all true physical explanations is a subset of the set of all true physical sentences, (3) and (4) entail that:

(5) Any of those physical explanations of the physical events involved in Napoleon's frowning which [by (1) and (3)] hold good in the actual world would still have held good if the world had been exactly as it is in all physically describable respects except that Napoleon had had no headache.

By now one might begin to doubt whether in the actual world Napoleon's headache is (as (1) has it) really a causal factor in the production of the physical events involved in his frowning. How is it possible for Napoleon's headache to be causally efficacious in the physical world if exactly the same physical explanations of the physical events in question would have been available even if he had had no headache? Indeed, how could the headache have made any difference at all to the course of physical events? It looks as if reference to Napoleon's headache is not required in order to explain those events. We shall eventually see that this is indeed the case. But to make the inference just now would be premature. There is a likely objection to be considered. True, the physical explanation of Napoleon's frowning does not refer to his headache in so many words (for by definition the explanation is composed wholly of physical sentences). But what if the headache has both physical and non-physical features? One might think that in that case the explanation could include physical descriptions which did in fact refer to Napoleon's headache in spite of the fact that the latter is not strictly implied by the physical sentences. On this view such physical descriptions would refer to the headache in the actual world (though not of course to its non-physical properties) even though they referred to no head-



FROM PHYSICAL EXPLICABIITY 233

ache in the possible world envisaged. In order to deal with this suggestion I now introduce the notion of a complete description or (as I shall call it) a specification.

A description Dx is a specification of a particular item x if and only if (a) Dx is non-relational in the sense defined earlier (that is, none of the truths about x that are strictly implied by Dx depends logically on the particular relations in which x stands to things external to it), and (b) Dx strictly implies all truths about x that are not excluded by (a). It follows from this definition that, armed with a specification of x, anyone with adequate ability and resources could, without knowing anything else about x, construct an exact replica of it-something exactly similar to x down to the minutest details. The world's complexity might be such that Dx would have to be infinitely long in some cases-or that a specification would have to consist of an infinite set of finite descriptions. (I owe this point to Robert Black.) But I think the argument could be adapted to take account of this possibility and for the sake of brevity I shall ignore it from now on. A physical specification will be a specification consisting wholly of words from the physical vocabulary (see section I).

Now there are two possibilities: Napoleon's headache either has, or has not, a physical specification. So let us first assume:

(6) Napoleon's headache has a physical specification Sx. We shall see that a consequence of including (6) among our other assumptions is that the true mental sentence 'Napoleon has a headache' is strictly implied by the set of all true physical sentences, contrary to assumption (2).

(7) It follows from (1) that any valid explanation of the physical events involved in Napoleon's frowning depends for its validity on a reference to Napoleon's headache. For if the headache could be bypassed in a valid explanation of those physical events, it cannot have made any contribution to bringing them about, contrary to (1).

(8) But since [by (1) and (3)] there is a valid physical explanation of the physical events involved in Napoleon's frowning, (7) entitles us to conclude that there is a physical explanation E of those events which depends for its validity on referring to Napoleon's headache.

(9) In this explanation E let all occurrences of expressions referring to Napoleon's headache be replaced by a variable. The resulting sentence Ex is an open physical sentence with one free variable. Then since [by (6)] Sx is a physical specification of Napoleon's headache, there is a true physical sentence s of the form '(3x) (Sx and Ex)'.

(10) Evidently this physical sentence s strictly implies that something, x, causes certain physical events, y. Moreover, it is a matter of fact that (i) any such item x is exactly similar to Napoleon's headache (because it answers to Sx); (ii) the events y are the physical events involved in Napoleon's frowning (because E explains those events); (iii) x causes y in exactly the same way as



234 ROBERT KIRK

Napoleon's headache causes y (because E explains how Napol- eon's headache causes y).

(11) Necessarily anything which is exactly similar to a headache is itself a headache, since 'headache' is a general term.

(12) Moreover, if some headache causes the physical events involved in Napoleon's frowning in just the same way as his headache causes those events, the former cannot fail to be a headache of Napoleon's: nobody else's headache could have caused his frown in just that way.

(13) Hence it is impossible, for linguistic and conceptual reasons, that the set of all actually true physical sentences-which of course includes s-should be true if 'Napoleon has a headache' is false. In other words, the set of all actually true physical sentences strictly implies that Napoleon has a headache, contrary to assumption (2).

We originally made three assumptions, (1), (2) and (3). It now turns out that the further assumption (6) (that Napoleon's headache has a physical specification) is inconsistent with (2) (that no true mental sentence describing Napoleon's headache is strictly implied by the set of all actually true physical sentences). So now let us see what follows if we assume, instead:

(14) Napoleon's headache has no physical specification. We saw at (5) above that any physical explanation of the physical events involved in Napoleon's frowning that was true in the world as it is would still have held good if the world had been exactly as it is in all physical respects, yet Napoleon had had no headache. This consequence looked as if it might rule out the possibility that physical explanations of those physical events depended for their validity on reference to Napoleon's headache. But we noted a possible objection to this inference: perhaps the headache has both physical and non-physical features or properties. In that case might not the physical explanations in question refer to the headache in the actual world, whilst referring only to physical items, not to a headache, in the possible world envisaged? By now it should be clear that this suggestion was misconceived. For if Napoleon's headache has no physical specification, no physical explanation of the physical events involved in his frowning can depend for its validity on referring to his headache. The reason is that exactly the same physical sentences are explanations of the same physical events both in the world as it is and in the world as envisaged, even though Napoleon has no headache in the latter case.

If any of these explanations had depended for its validity on referring to Napoleon's headache, it could not have retained its validity if there had been no headache to refer to. So:

(15) No physical. explanation of the physical events involved in Napol- eon's frowning depends for its validity on a reference to his headache.

Now by assumptions (1) and (3) there is at least one physical explanation of the physical events involved in Napoleon's frowning, from which it follows



FROM PHYSICAL EXPLICABILITY 235

by (15) that those events have at least one valid explanation F which (un- like E at step (8) above) does not depend for its validity on a reference to the headache. So F in effect shows that the headache has made no difference to the course of physical events: F bypasses the headache completely. But a valid explanation could not have done that if the headache had been a causal factor in bringing about the events in question. So we may conclude:

(16) The physical events involved in Napoleon's frowning are not an effect of his headache, contrary to assumption (1).

Given (1), (2) and (3), then, the assumption that Napoleon's headache has a physical specification entails a contradiction with (2), while the assumption that it has no physical specification entails a contradiction with (1). Since nothing hinges on the choice of example, it follows, as I set out to show, that:

(A) If a particular mental phenomenon has some physical effect, then some true mental sentence describing that phenomenon is strictly implied by the set of all actually true physical sentences.

III The next main step of the argument is:

(B) Most particular mental phenomena that are actually described have some physical effects.

Now (B) might seem uncontentious. It appears obvious that in general, if a particular mental phenomenon is described, it will have contributed to bringing about the physical events involved in the act of describing it. (There are exceptions. Sometimes we accurately guess others' states of mind without ourselves being affected by those states.) Yet it has recently been maintained that:

If [as the author holds] the brain's activities of a physical kind all occur in accordance with physical laws, suffering a burn, tasting the sweetness of sugar, or smelling the piquancy of cloves are pro- cesses in which the experience of the quality in question is inoperative in behaviour, even the behaviour in which such experiences are described (Keith Campbell, Body and Mind (London, 1970), p. 111).

Campbell accepts that there may be mental states which do not have these causally idle non-physical properties, and that the mental states which do have them also have physical properties which are causally efficacious. And he admits that the account of the causation of behaviour which he feels forced to accept is "rather paradoxical". Some would argue that it is not even coherent. But for my purposes I need only maintain that it is false. I shall mention just two considerations in support of this claim. First, Camp- bell's position entails that our actual experiences of headaches have had nothing whatever to do with the production and use of aspirins and other analgesics. This consequence seems to me to be obviously false. (When, at the end of section I, I alluded-perhaps rashly-to "something we all know to be true" it was the causal efficacy of our experiences of headaches that I had in mind.) A source of confusion here, I suspect, is the assumption



236 ROBERT KIRK

that the headache as a whole can have effects even if its non-physical properties-characterized as the "experience"-have none. My objection to this assumption is not just that the supposedly idle non-physical properties are (in the headache case) precisely those disagreeable experiences that aspirins are made to relieve. My chief objection is that this assumption is inconsistent for the reasons spelt out in steps (14)-(15) above. For what that part of the argument shows is that if headaches have non-physical features or properties, and therefore lack physical specifications, then it is inconsistent to maintain both that there are physical explanations of our making and taking aspirins and that our headaches-rather than headache- constituents which are not themselves headaches-contribute to the production of this behaviour.

The second consideration is this. The epiphenomenalist does not deny -how could he?-that we talk about the non-physical properties which he says are causally idle. But by saying this he deprives himself of any plausible explanation of the fact that our utterances refer to such properties- to the sweetness of sugar, the piquancy of cloves, the nastiness of headaches. On the one hand he insists that there is a possible world in which Napoleon has no headache but still utters sentences which, when uttered in the actual world, count as complaints about the headache. On the other hand he pro- vides no way of accounting for the fact that 'pain' refers to pains in the actual world but not in that possible world. Of course that is not the last word on the matter; but I shall say no more in defence of (B).

From (A) and (B) we may immediately conclude that:

(C) With few exceptions, for each particular mental phenomenon that is actually described, some true mental sentence describing it is strictly implied by the set of all actually true physical sentences.

Even if there are some mental phenomena that do not have physical effects, (B) entitles us to say that many mental phenomena, including mental phenomena of all the kinds that have been found most interesting and problematic, do have physical effects. And by (C), mental sentences describing those phenomena are strictly implied by the set of all true physical sentences. Now certainly we are not logically compelled to move from this to the conclusion that all true mental sentences are strictly implied by the set of all true physical sentences. One could consistently hold that some true sentences about pains, for example, are strictly implied by the physical facts, while others are not. But that would be a strange position to adopt. It would require one to hold that although in some cases the purely physical facts were enough to make a sentence ascribing pain to an individual true, in other cases the physical facts left logical room for it to be false. One would have to hold that in some cases the occurrence of a pain was a purely physical matter, while in other cases (which?) it involved the existence of something non-physical. This would be a gratuitous piece of metaphysical extravagance. If the physical facts strictly imply the existence of pains in



FROM PHYSICAL EXPLICABIITY 237

some cases, why suppose they are not sufficient to imply them in others? Nothing about the meaning of the word could require us to postulate special non-physical items in order to account for the truth of pain-descriptions in non-implied cases. Yet no other rational motive for postulating such non- physical items suggests itself. I conclude that one cannot sensibly accept (A) and not also accept that all true mental sentences are strictly implied by the set of all true physical sentences.

This conclusion still falls short of the Implication Thesis itself because the latter is to the effect that all true target sentences, not just the mental ones, are strictly implied by the physical facts. But if all true mental target sentences are strictly implied by the physical facts, I know of no reason to suppose that there are other sentences which are not so implied. I conclude that it would be unreasonable to accept (A) and deny the Implication Thesis.

Since all today's materialists accept the doctrine of Physical Explic- ability, the argument of this paper, if sound, shows that they at least cannot sensibly reject the Implication Thesis. This by itself would be a useful result if it is agreed that the Implication Thesis is reasonably clear. And of course this result holds independently of whether the doctrine of Physical Explicability is true. Yet, as I noted at the outset, the class of those who accept the doctrine of Physical Explicability is certainly larger than the class of those who would call themselves materialists. A number of philosophers who accept the former doctrine would regard the Implication Thesis as rather obviously false. So it seems worth having an argument which shows that this position is untenable. We must either give up the doctrine of Physical Explicability or accept materialism-and full-blooded materialism at that. And since such arguments as there are against the latter tend to rest on tenuous a priori considerations, while the case for the former is largely empirical-and gains strength as research in the neuro-sciences pro- ceeds-it seems to me that the argument of this paper must be regarded as supporting this strong form of materialism.

University of Nottingham



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