Despite their differences, the various forms of rationality share one important trait: they involve propositional attitudes, particularly belief and desire. These attitudes, and the ways in which they are typically described, raise a number of problems that have been the focus of attention not only in the philosophy of mind but also in logic and the philosophy of language. One particularly troublesome property of these attitudes is “intentionality”: they are “about things.” For example, the belief that cows are mammals is a belief about cows, and the belief that archangels are divine is a belief about archangels. In contrast, consider a star or a stone: on the face of it, it does not make sense to ask what they are about; stars and stones do not represent anything at all. But minds do. Beliefs, thoughts, feelings, and perceptions are all about something—they have “intentional content.” (Indeed, as noted above, this content is usually that of the sentential complement used to specify the attitude.)

Following medieval terminology, the German philosopher Franz Brentano (1838–1917) called this property of mental states intentionality. (This term is unfortunate, however, because intentionality in this sense has nothing specially to do with deliberate action, as in He did it intentionally. Many states that are intentional in Brentano’s sense can be unintentional in the ordinary sense.) Indeed, Brentano went so far as to propose that intentionality is a characteristic of all mental states and thus a mark of the mental. This idea is sometimes expressed as the claim that “consciousness is always consciousness of something.”

Of course, many of the peculiar products of minds—words, paintings, and gestures—also have content or are about things. The novel Moby Dick, for example, is about a great white whale. Such content, however, is usually derived from the mind or minds of the product’s creators or users; hence, it is called “derived” intentionality, as opposed to the “intrinsic,” or “original,” intentionality of mental states. One controversy about computers is whether the intentionality they display is original or merely derived.

Brentano noted a number of peculiarities about intentionality; two in particular are worth reviewing here.

1. Although intentional phenomena are about something, this “something” need not be real. People sometimes have thoughts about Santa Claus, the tooth fairy, or round squares—if only in thinking that they do not exist. Somehow, when people agree that Santa Claus does not exist, they are still thinking about the same thing. They are thinking thoughts with the same intentional content.

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.

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.