I begin this article, in Part 1, with a summary outline of the theses I have maintained in previous writings on this topic. In Part 2 (which has three sections, 2.1, 2.2 and 2.3), I treat the subject more fully, and with variations, reversing the order in which I approached the questions in those earlier writings. Finally, in Part 3, I try to set the whole discussion in the wider context of philosophy in general.