Mind, New Series, Vol. 84, No. 334. (Apr., 1975), pp. 255-259.
In Body and Mind (Garden City, N.Y., 1970) Keith Campbell argues that there is no dualist interaction between the mental and the physical. Given recent advances in brain physiology, he says, it is very likely that 'for explaining events in the brain, physiology is, in principle, complete' (p. 53). The completeness of physiology is, however, 'incompatible with matter-spirit interaction' (p. 40). Thus it is very likely that dualist interactionism is false. In this paper I hope to show that Campbell's argument does not succeed.
Campbell construes the completeness of physiology in at least three different ways. At times he envisages the possibility of explaining (and thus predicting and retrodicting) every brain happening using purely physical laws, i.e. laws referring only to physical states or events. Thus, given a complete physiology, every brain event would 'follow recognized physical laws' (p. 51), and no brain event would involve a 'departure from physical law' (p. 18). But if every brain event can be explained using purely physical laws, interactionism is, he claims, false: 'If 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 processes in which experience of the quality in question is inoperative in behaviour' (p. 111).
Even if it should prove possible to provide purely physical explanations of every brain event, however, it does not follow, as Campbell claims, that interactionism is false. Suppose a purely physical law correlating successive brain states of kinds B1 and B2, and having the form: b1 if, and only if, B2. Suppose, too, a psychophysical law correlating the simultaneous occurrence of mental states of kind M1 and brain states of kind B1, and having the form: M1 if, and only if, B1. For example, suppose that pain of a certain kind (M1) occurs if, and only if, the brain is simultaneously in a certain physical state (B1), and the brain is in that state if, and only if, it is subsequently in another particular kind of physical state (B2). Now in the situation supposed one can explain the occurrence of a B2 using only the purely physical law B1 if, and only if, B2. But one can also explain the occurrence of a B2 using the psycho-physical law M1 if, and only if, B2. One can explain the subsequent brain state by referring either to the prior brain state or to the prior pain (or to both, for that matter). Purely physical explanations are not, then, incompatible with psychophysical ones. It follows that the possibility of a complete physiology, in the present sense, does not rule out interactionist explanations of events in the brain. This is not, of course, to say that psychophysical explanations must be interactionist, for epiphenomenalism is a possibility here. More of this later.
At other times Campbell writes as though the progress of physiology suggests that brain events can be explained only in terms of purely physical laws. He writes: 'research based on the assumption that the brain obeys only physicochemical laws has not yet suffered a damaging reverse' (p. 39). If this were true, interactionist explanations would, of course, be impossible. But is there any reason to think that the advances of physiology will reveal that there are no psychophysical laws? Campbell certainly gives no defence of this rather implausible claim. Indeed, as an epiphenomenalist, he is himself committed to some sort of psycho-physical laws, as when he says that the 'burning hurtfulness' that a man experiences 'occurs only when he is in a particular brain state' (p. 110). Perhaps Campbell has simply confused the claim that one can explain every brain event using purely physical laws with the claim that one can only explain them in this way. As we have seen in the previous paragraph, however, the former claim does not entail the latter.
Most frequently Campbell takes the progress of physiology to reveal that the sole causes of any brain states are physical causes. The 'Shadow of Physiology', he says, suggests that 'all brain happenings are determined solely by physical influences operating on the antecedent physical condition of the brain' (p. 51). Even more explicitly he says: 'provided that neurophysiology is in principle complete, the only properties of the brain relevant to their role in causing behaviour will be physicochemical ones' (p. 110). If this is what the progress of physiology reveals Campbell could admit, though he does not do so, the possibility of psycho-physical explanation, while denying that of psychophysical causation of a non-epiphenomenalist sort.
It must be granted that if this is the result of further work in physiology interactionism is necessarily false. Difficulties arise, however, when one asks how the progress of physiology could show that brain states have only physical causes. To be sure, if purely physical laws should be discovered covering every brain happening, and no psychophysical laws, one would have reason to deny mental causation. But what if, as seems likely, one can establish psychophysical as well as physical laws? As we have seen, purely physical laws are compatible with psychophysical ones. In such a situation, the availability of purely physical laws would not, it seems, be sufficient of itself to rule out mental causation. More would be required than the evidence which the physiologist could gather.
Campbell says, truly, that if interactionism is true 'what happens in the mind must make a difference to what happens in the brain' (p. 51). Typically he assumes that, for the mind to make a difference, its physical effects must occur in violation of some physical law: if mental states are not (causally) 'idle, there must be departure from physical law' (p. 18). This, however, is an unwarranted assumption. An interactionist could assert that the mind makes a difference to what takes place in the brain, while admitting that what takes place in the brain conforms to physical laws. He need only say that a mental state of a man is a part cause, not the whole cause, of the subsequent state of his brain, that the mental state bears a one-one correlation to a co-temporaneous brain state, and that the co-temporaneous brain state is also only part of the cause of the subsequent state of the brain. In my earlier example, the present experience of pain is part cause of some subsequent state of the brain, but so too is that brain state which bears a one-one correlation to the experience of pain. On such a view, the pain does make a difference, for had it not occurred the subsequent brain state would not have occurred, (For the epiphenomenalist, the subsequent brain state would have occurred, given the prior brain state, even if the pain had not.) There would be no violation of physical law, however, given the one-one correlation of pain and present brain state.
That mental causation is possible without violation of physical laws is not, of course, sufficient to justify the claim that there is mental causation. For this one requires special evidence of some sort, not just logical possibility. Otherwise how decide between interactionism and epiphenomenalism? As Campbell has recognized in another connection, however, there is indeed evidence for mental causation. That mind and body interact is a proposition 'supported by an enormous and ever growing mass of common experience' (p. 34); 'we confirm the interaction of mind and body a hundred times a day' (p. 34). '[O]rdinary ways of searching for causes lead unambiguously to the conclusion that perceptions, decisions, emotions, and moods can all be causal antecedents of bodily actions' (p. 57). Assuming psychophysical correlation, then, one cannot reject interactionism in favour of epiphenomenalism on experiential grounds. Rather one must introduce more sophisticated considerations based on the requirements of scientific theory. Thus far, however, we have seen no such compelling theoretical objections to mind-body interaction.
The weakness of Campbell's argument from physiology is that he has adopted too narrow a view of dualist interaction. He recognizes that an interactionist need not postulate 'time-lags' or 'gaps' in the chain of physical causes in the brain. Nevertheless he assumes, as we have seen, that the theory requires the violation of physical laws. Connected with this, he assumes that, given the completeness of physiology, an alleged mental cause must be the whole cause of its effect, or no cause at all. This is evident from his claim that to avoid the anti-interactionist implications of physiology one must adopt the 'incoherent idea' of 'Double Causation, according to which both spiritual and material conditions are separate but complete causes of some particular brain events' (p. 52, my italics). Given these assumptions it is an easy matter to show that, if every brain event is capable of a purely physical explanation, interactionism is false. Giving up these assumptions, however, none of Campbell's arguments considered thus far can succeed. Nor, so far as I can see, is there any reason to grant Campbell's assumptions. Though there be no violation of physical law, and though mental causes not be the whole causes of physical events, one can make sense of the notion that the mind makes a difference to what happens in the brain.
At times, it seems, Campbell falls back on the claims of parsimony. If one can explain all brain happenings using purely physical laws, parsimony prohibits the employment of psychophysical laws. More importantly, if one can get on perfectly well with the assumption that brain happenings have only physical causes, parsimony forbids introducing non-physical ones. As Campbell writes:
[F]or most people researching brain function, the working hypothesis is that no such thing [as interaction of spirit and brain] occurs. For in the absence of evidence to the contrary, the most economical and therefore best assumption is that only physical causes are at work (pp. 54-55).
The economies in question are, it should be noted, of an explanatory or causal variety. Campbell's own epiphenomenalism rules out the more radical economies of Central State Materialism.
What shall we say of the argument from parsimony? The fact that a non-interactionist working-hypothesis is satisfactory for one's scientific purposes does not, of course, require the falsity of interactionism. At least two hypotheses can account for the success of the working-hypothesis : the truth of the working-hypothesis itself; or the truth of the sort of interactionism sketched earlier. Nevertheless, if there is no evidence to the contrary the simpler hypothesis is the better one. Several points should, however, be noted. As suggested earlier, it seems that there is evidence to the contrary: the evidence of ordinary experience. As Campbell has admitted, epiphenomenalism is a 'complicated and implausible position', whereas the 'initial plausibility' of interactionism is 'immense' (p. 35). Moreover, it seems that epiphenomenalism achieves only marginal economies when compared to the kind of interactionist theory introduced above. On either theory, one is faced with non-physical states or events. Economy is achieved only by denying a causal role to the apparently redundant non-physical states. The choice, then, is between a marginally more economical but implausible theory, and one which is marginally less economical but consonant with our common experience. Add to this the fact that if interactionism of the kind described is true there is no incompatibility between ordinary and physiological explanations of what goes on. We may say both that the pain caused the subsequent state of the brain and that the prior state of the brain was the cause of the subsequent one. Each statement would be elliptical, but each would be true. This has the virtue, as it seems to me, of according with our ordinary beliefs about such matters. The criterion of economy does not, I submit, point unambiguously in the direction of epiphenomenalism. I have argued that Campbell has failed to show either that the likely progress of physiology is incompatible with mind-body interaction, or that considerations of economy require us to adopt epiphenomenalism. Are there, however, other reasons that Campbell can and does offer for rejecting interaction? He mentions various other objections in the course of his discussion, most of them having to do with the anomalous character of psychophysical laws. "Such laws correlate phenomena of relative simplicity (mental states) with phenomena of staggering complexity (brain states), a feature of psychophysical laws not shared by any other scientific laws. Such laws can be neither fundamental nor derivative: not derivative, because not capable of revealing any causal mechanism; not fundamental because incapable of mathematical formulation. They cannot be causal laws because the alleged causes and effects are so different in character. An interactionist theory cannot give a satisfactory account of that point in the development of the individual or species at which a physical entity comes to have non-physical properties or states. Whatever the merits of these standard objections, however, they are not objections which Campbell can employ against mind-body interaction, for each carries equal weight against his own epiphenomenalism. Clearly Campbell's case must rest on the argument from physiology already considered. I conclude, then, that he has failed to show that dualist interactionism is false.
UNIVERSITY OF KANSAS