  A.J. Ayer (1910–1989) was only 24 when he wrote the book that made his philosophical name, Language, Truth, and Logic (hereafter LTL), published in 1936. In it he put forward what were understood to be the major theses of logical positivism, and so established himself as the leading English representative of the movement, Viennese in origin. In endorsing these views Ayer saw himself as continuing in the line of British empiricism established by John Locke and David Hume, an empiricism whose most recent representative was Bertrand Russell. Throughout his subsequent career he remained true to this tradition’s rejection of the possibility of synthetic a priori knowledge, and so he saw the method of philosophy to be the analysis of the meaning of key terms, such as ‘causality’, ‘truth’, ‘knowledge’, ‘freedom’, and so on. The major portion of his work was devoted to exploring different facets of our claims to knowledge, particularly perceptual knowledge and knowledge that depended on inductive inference for its credence. Along the way he defended a ‘justified true belief’ account of knowledge, a Humean account of causation, and compatibilism with respect to freedom.  In LTL he put forward an emotivist theory of ethics, one that he never abandoned.