This book, like most jointly written works, has arisen from a shared interest which we have developed together with mutual benefit and pleasure. We are especially grateful for the invention of the word-processor, which has stimulated continuous exchanges and criticisms by making correction pleasing rather than tedious, and which has, we think, resulted in a book which is a product of joint labour throughout, even in parts where one or other of us was originally responsible for more of the ingredients.

We have discussed the modes in several seminars and lectures, and we are grateful to our audiences for their patience and their helpful comments. We hope that the book will encourage further discussion, and that others may succeed in elucidating opaque argumentation where we have failed – and may even discover what Pericles' slave really was doing on the roof-top.