Perhaps everyone who can think has the concept of possibility, but no one understands it. The metaphysical theory of Determinism is a symptom of this lack of understanding, and the inconclusiveness of its opponents' arguments indicates that the lack is universal. In this book, first published in 1968, the author shows that there are a number of different kinds on non-logical possibility, subtly interrelated, each requiring separate explanation. An original contribution to the subject, it is essential reading for all students of philosophy.