The problem of natural kinds forms the busy crossroads where a number of larger problems meet: the problem of universals, the problem of induction and projectibility, the problem of natural laws and de re modalities, the problem of meaning and reference, the problem of intertheoretic reduction, the question of the aim of science, and the problem of scientific realism in general. Nor do these exhaust the list. Not surprisingly then, different writers confront a different ‘problem of natural kinds,’ depending on which background issue is for them the principal issue at stake. The issues of essentialism, meaning, and reference, for example, have tended to dominate recent discussions of natural kinds (Kripke [1972], Putnam [1975], [1981], Mellor [1977], Churchland [1979], Shapere [1982]). But evidently these are only part of the puzzle.

Consider a language SL having as its primitive signs one or more atomic statements, the two connectives ‘∼’ and ‘&,’ and the two parentheses ‘(’ and ‘)’; and presume the extra connectives ‘V’ and ‘≡’ defined in the customary manner. With the statements of SL substituting for sets, and the three connectives ‘∼,’ ‘&,’and ‘V’ substituting for the complementation, intersection, and union signs, the constraints that Kolmogorov places in [1] on (unary) probability functions come to read:

K1. 0 ≤ P(A),

K2. P(∼(A & ∼A)) = 1,

K3. If ⊦ ∼(A & B), then P(A ∨ B) = P(A) + P(B),

K4. If ⊦ A ≡ B, then P(A) = P(B).2

Ian Hacking's [6] is a spirited romp though a broad field of metaphysics, touching on a variety of important questions, and appealing to deep results in mathematical logic while remaining free of logical pedantry. Philosophical journals might be more fun to read if others could write in his style. It is an essay in applying the theory of logic expounded in more detail in his very interesting [7]. (Goran Sundholm, in [15], has raised grave difficulties for Hacking's notion of a ‘do-it-yourself’ semantics, but the overall project remains attractive. In particular, I find Hacking's contention that he has captured the notion of logic that Whitehead and Russell were working with both convincing and intriguing.) Unfortunately, despite my sympathy for his project, I have a number of criticisms to make of his argument. Some of them are picky logical points, but the major conclusions of [6] are affected.

It is widely held that we can obtain beliefs and withhold believing propositions directly by performing an act of will. This thesis is sometimes identified with the view that believing is a basic act, an act which is under our direct control. Descartes holds that the will is limitless in relation to belief acquisition and that we must be directly responsible for our beliefs, especially our false beliefs, for otherwise we could draw the blasphemous conclusion that God is responsible for them. For Descartes and his followers judgment and assent are acts of the will which may be made both when they ought and when they ought not to be made. They are expressions of freedom of the will and as such we are directly responsible for the beliefs we acquire. Other philosophers who seem to espouse volitionalism include Aquinas, Locke, Kierkegaard, Newman, James, Pieper, Chisholm and Meiland.

Leibniz has enjoyed a prominent place in the history of thought about possible worlds. I shall argue that on the leading interpretation of Leibniz's account of contingency - an ingenious interpretation with ample textual support - possible worlds may be invoked by Leibniz only on pain of inconsistency. Leibnizian contingency, as reconstructed in detail by Robert C. Sleigh, Jr., will be shown to preclude propositions with different truth-values in different possible worlds.

The conditional analysis of ability statements has many versions. In this paper I will deal with the version which claims that ‘x can do y’ is equivalent to ‘if x were to choose to do y, then x would do y.’ However, my comments should be equally applicable to any analysis of ability statements that can properly be called a version of the conditional analysis. The intuition behind the conditional analysis is that what it is for one to be able to do something is for one's choice to be effective. To have an ability to do y is for it to be true that one's choosing to do y would be effective - one's choosing to do y would result in one's doing y . But this intuition is not captured by the conditional analysis in its standard form, and a restriction is needed to mend this defect. This restriction is based on a distinction among counterfactual conditionals.

To paraphrase Freud, what do socialists really want? It is undoubtedly difficult to give a complete answer to this question that all socialists would be satisfied with, but there are some common elements that can hardly be denied. First and foremost among these is the elimination of capitalism; the elimination of capitalism would seem to require the elimination of capitalists (qua capitalists). Why might that be desirable? Well, many reasons might be offered, but one is suggested by the very nature of capitalism.