It was not long before her third birthday that one of my daughters saw her first hedgehog. The summer evening was warm and she had been put into the car for the drive home wearing only her pyjamas when she saw the hedgehog snuffling around next to a fence. She was so taken by it that we sat and waited while she watched the little fellow go about his business. One evening a few days later she insisted that she be dressed again in her pyjamas for the drive home, and was most disappointed when the hedgehog did not reappear, despite her efforts.

David Hume, renowned for his good-natured character, would have probably been quite amused by my daughter's naive conjecturing. However, he would probably also have pointed out that adults should not feel too self-satisfied, as our causal conjectures are not altogether different from the fumbling theorizing of children. In the two and a half centuries since Hume's death, philosophers have either sought to find ways to reject this conclusion or accepted it and claimed that, therefore, we do not possess knowledge about what the very next day will bring. One of the ways in which this book can be understood is as an attempt to show that accepting the problem Hume first recognized leads to no more than an explanation for the existence and, indeed, persistence of the very kind of supernatural beliefs and practices that he argued against in another context.