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.

What can we know about the world? How can we think and talk about it? Those two large questions determine two main areas of philosophy. Epistemology discusses questions of cognition: What is knowledge? How much can we know? Of what can we be certain? In what circumstances are our beliefs justified? Logic or philosophy of language is devoted to questions of meaning: What is it for us or our utterances to mean something? How can we refer to things in the external world? By what inferential processes can we legitimately move from one statement to another?

Among Anglo-Saxon philosophers, logic is now often taken to be the fundamental part of philosophy, and questions of meaning are accorded a certain priority: until we understand how we can talk and think about things we can make no progress. Such an attitude to philosophy is young. It traces its origins to the work of the German mathematician and logician, Gottlob Frege, whose active life spanned the period from 1880 to 1925.

Before the age of logic came the age of epistemology. For such thinkers as Descartes and Locke, Hume and Kant, the basic questions of philosophy concerned not language and thought but rather the nature and scope of human understanding. The first task of the philosopher was to determine in what ways and to what extent we can gain knowledge of the world. The triumph of logic has given a distinctive colouring to modern Anglo-Saxon philosophy: in style and approach, in method and argument, it differs noticeably from the philosophy of earlier epochs.

With this paragraph Sextus Empiricus, the Greek sceptical philosopher, begins his introductory handbook to sceptical thought, the Outlines of Pyrrhonism. He portrays sceptics as perpetual students or researchers, as people who ‘persist in their investigations’, and the Greek adjective skeptikos derives from a verb meaning ‘to inquire’ or ‘to consider’. Now inquirers persist in their inquiries because they have neither discovered the object of their search nor concluded that it lies beyond all discovery: they have, as yet, no opinion on the matter. Hence the word skeptikos or ‘sceptical’ acquires its familiar connotation. Sceptics are doubters: they neither believe nor disbelieve, neither affirm nor deny.

To be sceptical on any given matter is to suspend judgement on it, to subscribe to no positive opinion either way.