In modern philosophical literature there are two types of reasons for action, explanatory and justifying. The necessity of the distinction becomes clearer when we consider that reasons that explain may fail to justify, and that reasons that justify may fail to explain. For example, I explain that my reason for shouting at my boss was that she made me angry but this reason does not justify my action. Similarly, I might be justified in pursuing a healthy lifestyle but actually am motivated by winning the affection of the girl I am in love with. Furthermore, justifying reasons can apply to actions that are not performed at all and for which explanatory questions fail to even arise.
In addition to contexts of explanation and justification, we have contexts of deliberation, in which we look for and evaluate candidate-justifying reasons for doing or refraining from various actions; deliberative contexts provide a framework from within which we can determine what to do without necessarily having to act upon reasons to help guide our overall decision about whether to do something or not. Thus, our concern in such contexts is always normative rather than explanatory, though the reasons we determine for action or otherwise may subsequently be cited in explanation of what we do.
Importantly, the distinction between justifying and explanatory reasons is distinct in turn from the distinction between subjective and objective reasons. The latter distinction is actually two kinds of justifying reasons, either the reasons that apply to my circumstances as I, perhaps incorrectly, understand them (subjective) or the justifying reasons that apply to my circumstances as they actually are (objective).
Externalists adhere to a strict distinction between justifying and explanatory reasons. Internalists are charged with conflating the two together. The charge might be unfair to contemporary internalists such as Bernard Williams, however, who can be read as appreciating the distinction but questioning whether, if justifying reasons has little to do with what explains and motivates action (as externalists suppose), we are able to talk meaningfully about reason giving at all. For internalists, then, to talk of reasons is to talk about considerations that speak to our desires, either as they are or subject to some idealising constraint.
Desire-based theories of reason are appealing because they promise to make naturalistic sense of normativity, and thus explain why reason is of such interest and importance to us. However, making reasons contingent on facts about our motivation can have negative consequences, such as, if my motivational system is unusual enough, I might have reason for doing very odd things, or if moral concerns failed to speak to my motivational system then I would have no reason to act morally.
For some contemporary theorists, motivating reasons explain our actions via psychological states of an agent that make it possible to give a rational explanation for why the agent behaves in the way that he does. Motivational reasons contrast with normative reasons, which are more connected with whether an action is justified. For Michael Smith, following Davidson, for example, motivating reasons are complexes of beliefs and desires that motivate action and which can be cited in explaining them where the explanation in question is taken to be causal. Normative reasons, however, can also play an explanatory role, for example where beliefs about normative reasons motivate people who are practically rational.
For others such as Richard Norman, the theory of reasons outlined above is simply a redundancy theory where the sentence ‘x is motivated to §’ can be reduced to ‘x believes he has a reason to §’. As such, the phenomenon of acting in accordance with beliefs does not require a philosophical account of motivation to explain it. Rather, we simply offer a dispositional explanation along the lines of ‘human beings tend to do this’ just as we explain trees losing their leaves in autumn as ‘trees tending to do this’. Thus, the reasons why we ‘tend’ to do something are a matter for the empirical sciences, not a philosophical theory of motivation.