Two Humean accounts

Contemporary analyses of practical rationality generally adopt one of two accounts of how desires, reasons for action, and the explanation of action are related. The dominant view insists that something can be a reason for a person only if it relates in some way to his wants, desires, or pro- or con-attitudes. “[I]t is supposed to be a necessary condition of something's being a reason for an agent …,” writes E.J. Bond, “that it be a motivator, and motivations require wants (Hume's principle) in the broadest sense of that term. (No want, no reason).” According to this Humean orthodoxy, without a motivating desire or attitude one could never have more than a conditional reason to perform some act Φ: “there is a reason to Φ if you want to Φ or if Φ is a means to satisfying some other desire that you have.” So the desire believed to be necessary for a reason for action also provides the drive to get the agent moving. Reasons are then not merely justificatory, but, because they are causally effective, explanatory too.

Now it is clear that a reason cannot move a person to act unless it is what has been termed “an internal reason.” Though Smith's owing Jones $100 is a reason for Smith to give Jones $100, it is not a reason that can move Smith to pay up unless Smith believes that it is a reason for him.