The Causal Closure Argument
I begin by stating that I do not defend any form of materialism in this paper. Rather, I defend a commonsensical form of soul-body dualism in which souls make undetermined choices for purposes (reasons). I defend this commonsensical view of the world against an argument that is frequently used to undermine its truth. This is the argument from causal closure. Before setting forth and examining this argument, however, it behooves us to have a reasonably clear and concise commonsense sketch of how souls are causally related to their physical bodies on occasions when human beings make what I will assume are essentially undetermined choices (from here on, I will simply assume that choices are essentially undetermined). This picture is as follows: on certain occasions, we have reasons for performing incompatible actions. Because we cannot perform both actions, we must make a choice to do one or the other (or neither), and whichever choice we make, we make that choice for a reason or purpose, where that reason provides an ultimate and irreducible teleological explanation of that choice. The making of a choice is a mental event that occurs in a soul and either it, or some other mental event associated with it (e.g., an intention to act) directly causally produces an effect event in that soul’s physical body. In other words, there is mental-to-physical causation and its occurrence is ultimately and irreducibly explained teleologically by the reason that explains the making of the choice.
To put some flesh on the proverbial bones, consider the movements of my fingers right now on the keys of my keyboard as I work on this essay. If these movements occur because of a choice of mine to type, then these physical movements are ultimately and irreducibly explained teleologically in terms of the purpose for making my choice to write this essay, which, we can suppose, is that I make clear that there are no good scientific objections to the view that human beings are soul-body compounds and that those souls have free will (make choices for reasons). Hence, if the movements of my fingers are ultimately occurring because I made a choice to write this essay for a purpose, then a mental event involving me (a soul) must be causing those movements to occur as I write this essay for the purpose that I make clear that there are no good objections to the view that human beings have souls that make choices. In other words, if our commonsense view of a human being is correct, I, as a soul, cause events to occur in the physical world by making a choice to write this essay for a purpose.
From the example of my typing, it should be clear that the claim that there is causal interaction between a soul and its physical body is not a ‘God-of-the-gaps’ type of argument. In discussions about God’s existence, critics often argue that theists postulate God’s existence in light of an inability of science to provide a complete explanation for a physical datum (or data). This lack of a complete explanation is a gap in the scientific story. By analogy, a critic might argue that I am postulating my soul’s existence in light of an inability of science to provide a complete explanation for the movements of my fingers when I type this essay. But this argument would be mistaken. My claim is not that there are certain physical events (the movements of my fingers) for which a failure to find a complete physical causal story warrants appeal to the causal activity of a soul as their ultimate explanation. Rather, my claim is that our commonsense understanding of our purposeful activity entails that some physical events must occur whose ultimate causal explanation is not other physical events but non-physical mental events whose occurrences are explained teleologically by purposes.
What is wrong with this commonsense understanding of a human being? According to many philosophers, a serious problem for the view that souls make choices that causally produce events in physical bodies arises out of the practice of science.1 Richard Taylor puts forth a lengthy argument, the gist of which is as follows:Consider some clear and simple case of what would . . . constitute the action of the mind upon the body. Suppose, for example, that I am dwelling in my thought upon high and precarious places, all the while knowing that I am really safely ensconced in my armchair. I imagine, perhaps, that I am picking my way along a precipice and visualize the destruction that awaits me far below in case I make the smallest slip. Soon, simply as the result of these thoughts and images, . . . perspiration appears on the palms of my hands. Now here is surely a case, if there is any, of something purely mental . . . and outside the realm of physical nature bringing about observable physical changes. . . . Here, . . . one wants to say, the mind acts upon the body, producing perspiration.But what actually happens, alas, is not nearly so simple as this. To say that thoughts in the mind produce sweat on the hands is to simplify the situation so grossly as hardly to approximate any truth at all of what actually happens. . . . The perspiration . . . is secreted by tiny, complex glands in the skin. They are caused to secrete this substance, not by any mind acting on them, but by the contraction of little unstriated muscles. These tiny muscles are composed of numerous minute cells, wherein occur chemical reactions of the most baffling complexity. . . . These . . . connect eventually, and in the most dreadfully complicated way, with the hypothalamus, a delicate part of the brain that is centrally involved in the emotional reactions of the organism . . . . [B]ut it is not seriously considered by those who do know something about it that mental events must be included in the description of its operations. The hypothalamus, in turn, is closely connected with the cortex and subcortical areas of the brain, so that physical and chemical changes within these areas produce corresponding physical effects within the hypothalamus, which in turn, by a series of physical processes whose complexity has only barely been suggested, produces such remote effects as the secretion of perspiration on the surface of the hands.Such, in the barest outline, is something of the chemistry and physics of emotional perspiration. . . . The important point, however, is that in describing it as best we can, there is no need, at any stage, to introduce mental or nonphysical substances or reactions.2
According to Taylor, while we are inclined to believe that certain physical events in our bodies are ultimately explained by mental events of non-physical substances, as a matter of fact there is no need at any point to step outside of the physical causal story to explain the occurrences of those physical events. Jaegwon Kim uses an example of a neuroscientist to make the same point:You want [or choose] to raise your arm, and your arm goes up. Presumably, nerve impulses reaching appropriate muscles in your arm made those muscles contract, and that’s how the arm went up. And these nerve signals presumably originated in the activation of certain neurons in your brain. What caused those neurons to fire? We now have a quite detailed understanding of the process that leads to the firing of a neuron, in terms of complex electrochemical processes involving ions in the fluid inside and outside a neuron, differences in voltage across cell membranes, and so forth. All in all we seem to have a pretty good picture of the processes at this microlevel on the basis of the known laws of physics, chemistry, and biology. If the immaterial mind is going to cause a neuron to emit a signal (or prevent it from doing so), it must somehow intervene in these electrochemical processes. But how could that happen? At the very interface between the mental and the physical where direct and unmediated mind-body interaction takes place, the nonphysical mind must somehow influence the state of some molecules, perhaps by electrically charging them or nudging them this way or that way. Is this really conceivable? Surely the working neuroscientist does not believe that to have a complete understanding of these complex processes she needs to include in her account the workings of immaterial souls and how they influence the molecular processes involved. . . . Even if the idea of a soul’s influencing the motion of a molecule . . . were coherent, the postulation of such a causal agent would seem neither necessary nor helpful in understanding why and how our limbs move. . . . Most physicalists . . . accept the causal closure of the physical not only as a fundamental metaphysical doctrine but as an indispensable methodological presupposition of the physical sciences. . . . If the causal closure of the physical domain is to be respected, it seems prima facie that mental causation must be ruled out . . . .3
While Kim agrees with Taylor about the lack of a need on the part of a scientist to go outside the physical explanatory story, he introduces the stronger idea that to be successful the physical sciences need to make the methodological assumption of the causal closure of the physical world. Is he right about this? To insure clarity about what is at issue, consider one more example of movements of my body that according to common sense could only be adequately explained by mental causation of a soul whose choice is teleologically explained by a purpose or reason. Right now, I am tired and feel tight in my back after typing for several minutes, so I raise my arms in order to relax. Reference to my mental activity and my purposes for acting seems not only helpful but also necessary to explain both the movements of my fingers on the typewriter while I am typing and the subsequent motions of my arms when I relax. If we assume for the sake of discussion that I, as a soul, cause my fingers and arms to move by directly causing some neural events in the motor section of my brain, then when I move my fingers and raise my arms for purposes, I must directly cause initial neural events in my brain that ultimately lead to the movements of those extremities. In other words, in order to explain adequately (teleologically) the movements of my limbs, there must be causal openness or a causal gap in my brain. While Kim believes the commonsense view implies this causal openness, he also believes that it is because the commonsense view implies the existence of this causal gap that it must be mistaken. Because the neuroscientist methodologically assumes causal closure of the physical world, what she discovers as the explanation for what occurs in my brain and limbs when I type and relax must not and need not include reference to the mental causal activity of my soul and the ultimate and irreducible explanatory purpose for its choice to act. Given that the principle of causal closure entails the exclusion of a soul’s mental causation of a physical event and the ultimate and irreducible teleological explanation of that mental event and its effects by a purpose, it is imperative that we examine the argument from causal closure to see if it provides a good reason to believe that the movements of my fingers and arms when I am typing and stretching must be completely explicable in terms of neuroscience (or any other physical science), with the result that no reference to the causal activity of my soul and its purposes for typing and raising my arms is required.
Contrary to what Kim maintains, there is good reason to think that the argument from causal closure is unsound.4 To understand where it goes wrong, let us distinguish between a neuroscientist as an ordinary human being and a neuroscientist as a physical scientist. Surely a neuroscientist as an ordinary human being who is trying to understand how and why my fingers move and arms go up while I am typing must and would refer to me and my reasons (purposes) for acting in a complete account of why my limbs move.5 Must she, however, as a physical scientist, avoid making such a reference? Kim claims that she must avoid such a reference because as a physical scientist she must make a methodological assumption about the causal closure of the physical world. Is Kim right about this and, if he is, is such a commitment compatible with a commitment on the part of a physical scientist as an ordinary human being to causal openness? Or must a neuroscientist, who as a physical scientist assumes causal closure, also assume, if he is consistent, that as an ordinary human being his mention of choices and their teleological explanations is no more than an explanatory heuristic device that is necessary because of an epistemic gap in his knowledge concerning the physical causes of human behavior?
Read the rest of the argument.