METAPHYSICS Brown Book, where sections 58–67 are devoted to various language-games with the word ‘can’ The distinctions that he draws between processes and states, and between different kinds of states, correspond to the Aristotelian distinctions between kinesis, hexis, and energeia The criteria by which the two philosophers make the distinctions often coincide The example that Wittgenstein discusses at length, to illustrate the relation between a power and its exercise, namely learning to read (PI i 156 ff.), is close to the standard Aristotelian example of a mental hexis, namely, knowledge of grammar We may call the systematic study of actuality and potentiality dynamic metaphysics, and if we so we must say that Wittgenstein was one of the most consummate practitioners of that particular form of metaphysics It was not an Aristotelian type, however, but a Leibnizian one, that turned out to be the most flourishing version of metaphysics in the latter half of the twentieth century The development of modal semantics in terms of possible worlds5 need not, in itself, have had metaphysical implications, but a number of philosophers interpreted it in a metaphysical sense and were prepared to countenance the idea that there were identifiable individuals that had only possible and not actual existence In my view, this was a mistaken development There is a difficulty in providing a criterion of identity for merely possible objects If something is to be a subject of which we can make predications, it is essential that it shall be possible to tell in what circumstances two predications are made of that same subject Otherwise we shall never be able to apply the principle that contradictory predications should not be made of the same subject We have various complicated criteria by which we decide whether two statements are being made about the same actual man; by what criteria can we decide whether two statements are being made about the same possible man? These difficulties were entertainingly brought out by Quine in his famous paper ‘On What There Is’ of 1961: Take, for instance, the possible fat man in that doorway; and again, the possible bald man in that doorway Are they the same possible man, or two possible men? How we decide? How many possible men are there in that doorway? Are there more possible thin ones than fat ones? How many of them are alike? Are no two possible things alike? Is this the same as saying that it is impossible for two things to See p 119 above 189