It is important to distinguish such claims about the dualistic nature of mental phenomena from claims about their causal relations. In Descartes’s view, mental phenomena, despite their immateriality, can be both causes and effects of physical phenomena (“dualistic interactionism”). The dualist does not ipso facto deny that physical phenomena in the brain quite regularly cause events in the mind and vice versa; he merely denies that those phenomena are identical to anything physical.

A problem with dualistic interactionism, however, concerns the evident lack of any causal break in the internal processes of the human body. So far as is known, there is no particular state of any part of the body—no action of any muscle, no secretion of any substance, no change in any cell—that cannot in principle be explained by existing physical theories, assuming it can be explained at all (quantum indeterminacy is irrelevant to the present point). Serious evidence of so-called “paranormal” phenomena, such as telepathy, is yet to be found. More generally, there seems to be very good reason to think that the physical world forms a closed system, obeying conservation laws such as the conservation of mass and the conservation of energy. Consequently, there would appear to be no explanatory need to introduce nonphysical phenomena, whether substances or properties, into any account of human activities. (In contrast, before the introduction of electromagnetism in the late 19th century, there were myriad phenomena that could not be explained without supposing the existence of another force in addition to gravitation.)

In response to this difficulty, dualists have tried to exempt the mental from any causal role. Leibniz claimed that mental events were neither causes nor effects of any physical events—they were simply “synchronized” by God with physical phenomena, a view known as “parallelism.” A more moderate position, originally advocated by the English biologist T.H. Huxley (1825–95) and revived by the Australian philosopher Frank Jackson in the late 20th century, is that mental phenomena are the effects, but not the causes, of physical phenomena. Known as “epiphenomenalism,” this view allows for the evident causal laws relating physical stimuli and perceptual experiences but does not commit the dualist to claims that might conflict with the closure of physics.

These responses, however, may serve only to make the problem worse. If the mental really does not have any effects, then it becomes entirely unclear why one should believe that it exists. What possible reason could there be for believing in the existence of something in the spatiotemporal world that does not affect anything in that world in any way? Epiphenomenal mental phenomena would seem to be no different in this respect from epiphenomenal angels who accompany the planets without actually pushing them. At this point it becomes hard to resist the invitation that dualism extends to eliminativism, the view that mental phenomena do not exist at all.

The need for nontendentious evidence

One might be tempted to dismiss at least a blanket eliminativist view that denies the reality of any mental phenomenon by asking how any such theory could explain one’s own present conscious thoughts and experiences. But here it is crucial to keep in mind a principle that should be observed in any rational debate: in arguing against a position, one must not presuppose claims that the position explicitly denies. Otherwise, one is simply begging the question. Thus, it is no argument against a Newtonian account of planetary motion that it does not explain the fluttering of the angelic planet pushers’ wings, since precisely what the Newtonian account denies is that one needs to posit angels to explain planetary motion. Similarly, it is no argument against someone who denies mental phenomena that his view does not explain conscious experiences. “What conscious experiences?” the eliminativist might ask. What is needed in defending the existence of either angels or mental states is nontendentious data for the postulation in question.

This is, at first blush, a difficult challenge to meet. It is not obvious what nontendentious evidence for the existence of minds could consist of; indeed, their existence is actually presupposed by some of the evidence one might be tempted to cite, such as one’s own thoughts and other people’s deliberate actions. However, nontendentious evidence can be provided, and regularly is.

Consider standardized aptitude tests, such as the Scholastic Assessment Test (SAT) and the Graduate Record Examination (GRE), which are regularly administered to high school and college students in the United States. Here the standardization consists of the fact that both the question sheets and the answer sheets are prepared so as to be physically type-identical—i.e., the question sheets consist of identically printed marks on paper, and the answer sheets consist of identically printed rectangles that are supposed to be filled in with a graphite pencil, thus permitting a machine to score the test. Consider now the question sheets and the completed answer sheets that make up a single test that has been administered to millions of students at about the same time. The observable correlations between the printed marks on the question sheets and the graphite patterns on the answer sheets will be, from any scientific point of view, staggering. Overwhelmingly, students will have produced approximately the same graphite patterns in response to the same printed marks. Of course, the correlations will not be perfect—in fact, the answer sheets are supposed to differ from each other in ways that indicate likely differences in the students’ academic abilities. Still, the correlations will be well above any reasonable standard of statistical significance. The problem for the eliminativist is how to explain these standardized regularities without appealing to putative facts about the test takers’ mental lives—i.e., to facts about their thoughts, desires, and reasoning abilities.

Here it is important to remember that, in general, what science is in the business of explaining is not this or that particular event (or event token) but the regularities that obtain between different kinds of events (or event types). Although the fact that every token physical movement of every test taker is explainable in principle by physical theories, this is not in itself a guarantee that the types of events that appear in these correlations can also be so explained. In the case of standardized regularities, it is hard to think of any purely physical explanation that stands a chance.