There are two main questions which the theory of knowledge is designed to answer, namely ‘What is knowledge?’ and ‘How, if at all, can our ordinary claims to knowledge be justified?’ Ayer’s predominant concern is with the second of these questions. Or rather, he is concerned with this second question as part of his predominant concern with the more general question of how, if at all, our ordinary beliefs can be justified. His concern with this question is prompted by the challenge of the philosophical sceptic, who claims that most of our ordinary beliefs cannot be rationally defended.