The “two cultures” is a phrase invented by C. P. Snow in 1959,1 but the phenomenon he was describing is of course older. It is not, however, all that old, and that is the point of this book. In terms of human history, the idea that there are two cultures is relatively recent. Furthermore, whereas when Snow wrote his book, the concept seemed self-evident, which is why Snow’s phrase caught on, its validity has come under increasing challenge since the 1960s. We shall try to explain the origin of the concept, its impact and pervasiveness, and the nature of the challenges to it in recent years. This will, we hope, enable us to assess the likelihood that this way of structuring knowledge will continue to prevail and, if it does not, what the alternatives are.