No alterations have been made in the text of the original edition for the purposes of the present printing; but I am glad of the opportunity to say that, five years after the original publication, I still feel that the questions raised in the present book are as relevant to the main themes of current British philosophy as they were when the book was first written. The reception which the argument of the book met with from the critics in fact served only to sharpen for me the point of my central thesis—namely, the contrast between the standards and values of practical reasoning (developed with an eye to what I called ‘substantial’ considerations) and the abstract and formal criteria relied on in mathematical logic and much of twentieth-century epistemology. The book has in fact been most warmly welcomed by those whose interest in reasoning and argumentation has had some specific practical starting-point: students of jurisprudence, the physical sciences, and psychology, among others. Whether the implications of my argument for logical theory and philosophical analysis will become any more acceptable with the passage of time remains to be seen.