2. Intentional content seems to play a role in people’s thoughts even when it is about a real object in the world, since people can associate different intentional contents with the same real object. The German logician Gottlob Frege (1848–1924) noted that, from the fact that someone thinks that the morning star is Venus, it does not follow that he thinks that the evening star is, even though the morning star and the evening star are one and the same thing (Venus). Indeed, in general one needs to be very careful about substituting words that refer to the same thing in the complement clauses of propositional-attitude verbs, since doing so can affect the validity of the inferences in which the sentences are involved. As the American philosopher W.V.O. Quine (1908–2000) discussed in some detail, such verbs are “referentially opaque,” and this feature seems to be a peculiar manifestation of the intentionality of the states they describe. In contrast, most verbs, such as visit, are “referentially transparent”; if someone visits the morning star, then it follows that he visits the evening star.
Gottlob Frege.Courtesy of the Universitatsbibliothek, Jena, Ger.
These and other peculiarities led Brentano to be deeply pessimistic about the possibility of explaining intentionality in physical terms, or “reducing” it to the physical, a view that has come to be called Brentano’s thesis (see below Reductionism). Despite the concerted efforts of many philosophers during and since the 20th century, no one has succeeded in refuting Brentano’s thesis (see below Research strategies for intentionality). Peripheral issues
There are two issues that were once central to the philosophy of mind but are now somewhat peripheral to it, though they still command a great deal of philosophical attention. They are the problem of free will, also called the problem of freedom and determinism, and the problem of whether a person’s mind can survive his death. Free will
A problem that dates to at least the Middle Ages is that of whether a person’s moral responsibility for an action is undermined by an omniscient God’s foreknowledge of his performance of that action. If God knows in advance that a person is going to sin, how could the person possibly be free to resist? With the rise of modern science, the problem came to be expressed in terms of determinism, or the view that any future state of the universe is logically determined by its initial state (i.e., the big bang) and the laws of physics. If such determinism is true, how could anyone be free to do other than what physics and the initial state determined?
Although this problem obviously has much to do with the philosophy of mind, it is less important than it used to be, in part because there are already so many problematic mental phenomena that need not involve free will; conscious and intentional states, for example, often occur quite independently of issues of choice. Moreover, many aspects of the problem can be seen as instances of certain more general issues in metaphysics, particularly issues regarding the logic of counterfactual statements (statements about what might have happened but did not) and the nature of causality and determinism. The soul and personal identity
Perhaps the problem that most people think of first when they think about the nature of the mind is whether the mind can survive the death of the body. The possibility that it can is, of course, central to many religious doctrines, and it played an explicit role in Descartes’s formulations of mind-body dualism, the view that mind and body constitute fundamentally different substances (see below Substance dualism and property dualism). However, it would be a serious mistake to think that contemporary controversies about the nature of the mind really turn on this remote possibility. Although it can often seem as though debates between dualism and reductionism—the view that, in some sense, all mental phenomena are “nothing but” physical phenomena—are about the existence of disembodied spirits, virtually none of the contemporary forms of these disputes take this possibility seriously, and for good reason: there is simply no serious evidence that anyone’s mind has ever survived the complete dissolution of his body. Purported “out of body” experiences, as well as people’s alleged memories of events occurring minutes after they are pronounced dead, are no more evidence of disembodiment than are the dreams that many people have of witnessing themselves doing various things.
There is, however, an interesting problem related to the question of disembodied souls, one that can be raised even for someone who does not believe in that possibility: the problem of personal identity. What makes someone the same person over time? Is it the persistence of the same body? Suppose that the cells in one’s body became diseased and that it was medically possible to replace them one-by-one with new cells. Arguably, if the replacement was extensive enough, one would be the same person with a new body. And it is presumably something like this possibility that people envision when they imagine reincarnation.
But if it is not the body that is essential to being the same person, then it must be something more purely psychological—perhaps one’s memories and character traits. But these come and go over a lifetime; most people remember very little of their early childhoods, and some people have trouble remembering what they did only a week earlier. Certainly, many of one’s interests and character traits change as one matures from childhood to adolescence and then to early, middle, and late adulthood. So what stays the same? In particular, what is it that underlies the peculiar concern and attachment one feels about the even distant future and past portions of one’s life? It is not at all easy to say. (Note that this is a problem as much for the believer in immaterial souls as for the person who believes only in bodies. To vary the story by Mark Twain, how would it be possible, without some criterion of personal identity, to distinguish the case of a prince being reincarnated as a pauper from the simple case of a pauper being born shortly after the death of a prince?) Traditional metaphysical positions
A good deal of traditional discussion in the philosophy of mind is concerned with the so-called mind-body problem, or the problem of how to explain the relation of mental phenomena to physical phenomena. In particular, how could any mere material thing possibly display the phenomenon of intentionality, rationality, or consciousness?
Interest in this issue is not confined merely to those with a penchant for physics. On pain of circularity, if mental phenomena are ultimately to be explained in any way at all, they must be explained in terms of nonmental phenomena, and it is a significant fact that all known nonmental phenomena are physical. In any case, as noted above, reductionism—also called materialism or physicalism—is the view that all mental phenomena are nothing but physical phenomena. Reductionism The identity theory
The simplest proposal for explaining how the mental is nothing but the physical is the identity theory. In his classic paper “Materialism” (1963), the Australian philosopher J.J.C. Smart proposed that every mental state is identical to a physical state in the same way, for example, that episodes of lightning are identical to episodes of electrical discharge. The primary argument for this view is that it enables a kind of economy in one’s account of the different kinds of things in the world, as well as a unification of causal claims: mental events enter into causal relations with physical ones because in the end they are physical events themselves. This view is also called reductionism, which unfortunately conveys the misleading suggestion that the mental is somehow “made less” by being physical. This is of course mistaken, as lightning is no less lightning for being reduced to electrical discharge, and water is no less water for being reduced to H2O.