Kant is famous for undertaking a critique of reason and for calling two of his most significant works critiques of reason. These titles raise suspicions. Does Kant genuinely criticise reason, thereby calling into question the very processes by which any reasoned thought or action – including any criticism of reason – should be conducted? Or does he give these pretentious titles to works that deploy rather than criticise reason? Indeed, could anything really, seriously count either as a critique of reason or as a vindication of reason? Isn't the very idea that we could show that certain ways of thinking or acting are reasoned or reasonable absurd? After all, the demonstration must either build on assumptions that lack reasoned vindication, or be supported by arguments that deploy the very conception of reason supposedly vindicated. So it will be either unreasoned or circular: either way it will fail to vindicate reason. We have grounds for suspecting that no ways of organising thinking or acting have unconditional authority, and that Kant cannot have vindicated reason.

Kant's attempt to give an account of practical reason that offers unconditional reasons for action and provides the basis for a reasoned account of human duties is spectacularly ambitious: even if it fails in some ways it is worth the closest attention. Here I aim to give as coherent account of that attempt as I can offer, although I shall say nothing about the connections Kant draws between practical and theoretical reason. Since practical reasoning aims to shape and select action, I begin with a short account of Kant's views on action.

Practical reasoning and the agent perspective

Agents use practical reasoning to shape or guide their future action. Since practical reason has to bear on action yet to be done, it cannot bear on act tokens: there are no relevant, individuable act tokens at the time that practical reasoning takes place. So practical reasoning has to bear on act types (including types of attitude). It might be used to provide reasons for thinking that certain types of action or attitude are required or forbidden, recommended or inadvisable.

As Kant sees it, types of action are specified by act descriptions, while normative claims are expressed in principles that incorporate act descriptions.