 Humans engage in practical reasoning: they deliberate about what to do and how to do it. And they often act in light of reasons which can then explain their actions, and may also justify them. These ideas go back to Plato (Protagoras and Republic, Book 4) and Aristotle (De Anima, see esp. III.10; see also Price 2011). They have been a constant theme in discussions of the character of human behaviour in the history of philosophy. In the 18th century, David Hume and Immanuel Kant offered radically different views about the role and importance of Reason (the faculty of reason) in guiding and justifying human actions. Their contributions remain influential today, but in the second half of the 20th century, the focus shifted from discussion of the faculty of reason to discussion of the very concept of a reason and to questions about different kinds of reasons and their interconnections.