Michael Moore defends an account of scientific, mental, moral, and legal properties, according to which there are not only natural kinds, but also moral and functional kinds; and he maintains, more specifically, (1) that distinctively legal phenomena, such as legal rights, legal duties, precedent, malice, etc. are functional kinds, in the sense that they have a nature that consists not in their structure, but in the function they fulfill in law; (2) that the function of a functional kind is that effect, or those effects, of the functional kind that causally contribute more than does any of its other effects to the goal of the larger system within which it occurs; and (3) that functional kinds can be reduced to indefinitely large disjunctions of natural properties, (4) that the relevant version of naturalism is metaphysically reductionist naturalism, and (5) that functional kinds play an indispensable role in the explanation of human behavior.

      I argue, however, (i) that the method for determining the function of a (purported) functional kind proposed by Moore is too indeterminate to be able to pin down the function in an unequivocal way; that this suggests, though it does not entail, that there is no fact of the matter as regards the question of what the function is of a functional kind, and that there seems to be no way to remedy this troublesome indeterminacy. I also argue – assuming, for the sake of argument, that functional kinds do exist and that we can determine their function using Moore’s method – (ii) that it turns out to be very difficult to identify the properties that are part of the indefinitely large disjunction of natural properties which, on Moore’s analysis, is identical to a functional kind;  (iii) that functional kinds cannot be part of the best explanation of human behavior, because they lack systematic (or nomological) unity, and that they lack such unity because they are necessarily multiply realizable, in the sense that they can be realized on different occasions by different first-order properties that may have very different causal powers; and (iv) that Moore will therefore have to give up (a) the view that functional kinds are identical to indefinitely large disjunctions of natural properties, or (b) the view that functional kinds are part of the best explanation of human behavior, or both; that if he drops (a), his naturalism will be undermined, or at least seriously threatened, and that if he drops (b), he will have to admit either that functional kinds do not exist, or that they are not as interesting as they seem. On the basis of these claims, I also argue (v) that the idea of a functional kind should not play a central role in any theory of law or legal reasoning, or in any other theory of a legal phenomenon.