1. For accounts of Humean instrumentalism see ch. 3 of my Practical Reasoning (London and New York: Routledge, 1989), Richard Fumerton, Reason and Morality (Ithaca: Cornell University Press, 1990), esp. chs 4-6, and Garrett Cullity and Berys Gaut (eds) Ethics and Practical Reason (Oxford: Oxford University Press, 1997). For a broad characterization of instrumentalism and responses to it, see the introduction by Cullity and Gaut. The papers by James Dreier - 'Humean Doubts about the Practical Justification of Morality' - and Peter Railton - On the Hypothetical and Non-Hypothetical in Reasoning about Belief and Action' - defend certain elements of instrumentalism, and Dreier's brings out the limitations of instrumentalism as a route to providing reasons to be moral (or alternatively, 'justifying morality'). Cullity's 'Practical Theory' critically examines Bernard Williams's well-known version tracing to his much-discussed 'Internal and External Reasons', in his Moral Luck (Cambridge: Cambridge University Press, 1981); and Gaut's "The Structure of Practical Reason' assesses a Kantian constructivist response to instrumentalism (represented in the volume by Christine Korsgaard's paper), and sketches a broadly Aristotelian alternative to both.
2. These five kinds of reasons are introduced and discussed in my 'Acting for Reasons', Philosophical Review, 95 (1986), reprinted in my Action, Intention, and Reason (Ithaca and London: Cornell University Press, 1993) and, in a different way, in The Architecture of Reason (Oxford and New York: 2001), esp. ch. 5, on which this paper draws significantly. I might add that since motivating reasons as here described operate in producing or sustaining action, one might also call them activating reasons; and since subjective reasons may or may not activate behaviour, but are the appropriate kind to motivate it, one could call them motivational as opposed to motivating. We could also distinguish a sixth variety: inclining reasons, those that produce a tendency to act, and are in that sense (causally) operative, but do not yield action itself. But for our purposes there is no need to complicate the terminology in the text.
3. See A Treatise of Human Nature, ed. L. A. Selby-Bigge (Oxford: Oxford University Press, 1888). The famous claim is on p. 415, the point about strength of mind on p. 418.
4. Two points of clarification will help. First, I assume for the sake of argument that we may speak of combined desire strengths at least in a way that enables us to make comparative determinations of strength. Second, my focus is the rationality of an action for S, not S's acting rationally; I take the latter to be more complex, requiring that the action be based on the reason(s) in virtue of which it is rational. Supporting argument on the latter point is given in my 'Rationalization and Rationality', in my The Structure of Justification (Cambridge and New York: Cambridge University Press, 1993). Detailed explication of the instrumentalist notion of desire satisfaction, and indeed of rational action in general, is provided by Fumerton, op. cit., esp. ch. 4.
5. For a plausible attempt to present a detailed naturalistic epistemology see Alvin I. Goldman, Epistemology and Cognition (Cambridge, MA: Harvard University Press, 1986). I have critically appraised the attempt to naturalize epistemology in The Structure of Justification, esp. chs 6, 10, and 12, and in 'An Internalist Theory of Normative Grounds', forthcoming in Philosophical Topics.