Before we can begin to think about Hume’s views on causation, we need to focus on some preliminary issues. As we saw in Chapter 1, Hume describes his project as the ‘science of man’, and to that end, at the beginning of the Treatise and the Enquiry, he begins to develop his theory of how the mind works. Central to that theory is the notion of a ‘relation’ between ideas; and by far the most important relation between ideas for Hume, at least as far as the beliefs that enable us to go about our daily lives and to conduct experimental investigations are concerned, is causation, since ‘[b]y means of that relation alone we can go beyond the evidence of our memory and senses’ (E 26). Hume’s account of how it is that we ‘reason’ from causes to effects and vice versa cannot, however, be understood in isolation from his account of other kinds of reasoning, in particular his account of a priori reasoning – the kind of reasoning that delivers knowledge that 4>2, for example, or that the angles of a triangle add up to 180°. Nor can his views on causation be understood in isolation from his distinction between ‘natural’ and ‘philosophical’ relations – a distinction that is crucial for understanding his much-discussed ‘two definitions’ of causation. These important building blocks of Hume’s science of man are the main focus of this chapter, and his account of ‘reasoning’ from causes to effects is the main focus of Chapter 3.