There are two reasons which could be given for putting together a collection of this sort. The first is that some of these studies, while still in demand, are out of print or not easily accessible; the second, that there may be some interest in placing an author's writings side by side, so that the connections between them, and the way in which his views have developed, may be more easily traced. The first requires no further explanation. But, in the case of the second, some guidance to the various themes discussed, with some comment on their development, and to the cross-references between them from one chapter to another may be helpful; the more so because the items included in the collection are printed in their historical sequence, and not in the order of composition. If the latter were adopted, Chapter 1 would take the place of Chapter 8 as the penultimate in the book; and Chapters 2 and 3 would be transposed. The Introduction aims therefore to provide a preliminary guiding commentary, easily dispensed with by anyone who does not find such a thing at all necessary. With the exception of Chapter 9, which has not been previously published, the various items are printed here more or less as they first appeared, with no attempt at significant revision.