British philosopher and educator and a leading representative of Logical Positivism through his widely read work Language, Truth, and Logic (1936). Although Ayer's views changed considerably after the 1930s, becoming more moderate and increasingly subtle, he remained loyal to Empiricism, convinced that all knowledge of the world derives from sense experience and that nothing in experience justifies a belief in God or in any other extravagant metaphysical entity. His logical views alone, expressed in an elegant, crystalline prose, would have ensured him a place in the history of modern philosophy. But Ayer, playful and gregarious, was also a brilliant lecturer, a gifted teacher, and a successful broadcaster, as ready to offer his opinions on politics and sports as on logic and ethics. Named a fellow of the British Academy in 1952 and knighted in 1970, he became one of the most influential British philosophers of the 20th century.

Although Ayer was raised in London, both his father, a French Swiss businessman, and his mother, a Dutch citizen of Jewish ancestry, were born abroad, and Ayer grew up speaking French fluently. An extremely able, though sensitive, boy, he won a scholarship to Eton College (1923), where he excelled in classics but had no opportunity to study science, an omission that he would always regret. In 1929 he won a classics scholarship to the University of Oxford, where he also studied philosophy. His tutor, Gilbert Ryle (Ryle, Gilbert) (1900–76), soon described Ayer as “the best student I have yet been taught by.” While at Eton, Ayer had read essays by Bertrand Russell (Russell, Bertrand) (1872–1970), one of which, "On the Value of Scepticism" (1928), proposed a “wildly paradoxical and subversive” doctrine that Ayer would adopt as a lifelong philosophical motto: “It is undesirable to believe a proposition when there is no ground whatever for supposing it true.” At Oxford, Ayer studied A Treatise of Human Nature (1739) by the radical empiricist David Hume (Hume, David) (1711–76) and discovered the recently published Tractatus Logico-Philosophicus (1921) by Ludwig Wittgenstein (Wittgenstein, Ludwig) (1889–1951). Instinctively irreverential, he used both works to attack the conventionally religious, socially conservative figures who then dominated philosophy at Oxford.

Having secured a fellowship at the college of Christ Church, Ayer spent part of 1933 in Vienna, where he attended meetings of the Vienna Circle, a group of mostly German and Austrian philosophers and scientists who were just then beginning to attract the attention of philosophers in England and the United States. Although Ayer spoke poor German and was hardly able to take part in the discussions, he became convinced that the doctrine of logical positivism that the group was developing marked an important advance in the empiricist tradition, and he returned home an ardent convert. Within two and a half years he had written a manifesto for the movement, Language, Truth, and Logic.

In this work, following Wittgenstein and the members of the Vienna Circle, Ayer defended a verificationist theory of meaning (also called the verifiability principle), according to which an utterance is meaningful only if it expresses a proposition the truth or falsehood of which can be verified (at least in principle) through experience. He used this theory to argue that metaphysical talk about God, the cosmos, or “transcendent values” was not merely, as earlier empiricists had maintained, excessively conjectural but literally meaningless. Ayer's specific contribution was to develop this argument with unusual clearheadedness and rigour, showing how statements about the external world, other minds (other minds, problem of), and the past could be accorded sense through an analysis in verificationist terms. His argument that statements of moral evaluation, because they are unverifiable, are not descriptions of fact but merely “emotive” expressions of feeling aroused particular controversy.

Language, Truth, and Logic eventually became one of the best-selling works of serious 20th-century philosophy. Despite his obvious abilities, however, Ayer could not secure a permanent teaching position at Oxford—a fact that increased his hostility toward the philosophical establishment there.

Although Ayer claimed that Language, Truth, and Logic answered all major philosophical questions, the problems he had so confidently “solved” soon came back to haunt him. In a series of important papers and a book, The Foundations of Empirical Knowledge (1940), he wrestled with critics who doubted that all meaningful discourse could be analyzed in terms of sense experience. In particular, he turned for the first time to a careful analysis of the “sense-data” that empiricists had always claimed were the basis of all real knowledge. In a characteristic move, Ayer now contended that sense-data should be understood not as part of the furniture of the world but as conceptual constructs, or logical fictions, that allow one to distinguish between sensory experience and matter and between appearance and reality.

In the years surrounding the publication of Language, Truth, and Logic, philosophy had to compete with more pressing concerns. Like many young men of the period, Ayer was critical of what he saw as the British government's do-nothing approach to the rise of unemployment at home and of fascism abroad. After briefly considering joining the British Communist Party, Ayer instead joined the Labour Party. An early and forthright critic of Neville Chamberlain (Chamberlain, Neville)'s policy of appeasement, Ayer volunteered for the Welsh Guards as soon as war broke out. After completing officer training, he joined an intelligence unit, eventually becoming a specialist on France and the French resistance and gaining the rank of major. His war assignments took him to New York, to Algeria, and, after the liberation of France, to the southern part of that country and to Paris.

At the end of the war, Ayer at last secured an Oxford fellowship. One year later, in 1946, he was appointed Grote Professor of Mental Philosophy at University College, London. Although little philosophy had been published in England during the war, Ayer found that the philosophical climate was now very different. Influenced by the ideas of the later Wittgenstein, which were only then becoming known outside Cambridge, a group of philosophers at Oxford, led by Gilbert Ryle and J.L. Austin (Austin, John Langshaw) (1911–60), were arguing persuasively that most philosophical problems were simply conceptual confusions resulting from philosophers' insufficient attention to the complex ways in which philosophically loaded terms and their cognates were used in ordinary speech (see ordinary language analysis). Although Ayer well understood the Oxford philosophers' weariness with metaphysical speculation and supported their commitment to careful conceptual analysis, he did not share their hostility toward philosophical theorizing. He remained loyal to the outlook of Russell, the early Wittgenstein, and the logical positivists and admired American philosophers such as W.V.O. Quine (Quine, Willard Van Orman) (1912–2000), who, like Ayer, attempted to build on the positivists' work.

The next decade and a half, until the early 1960s, was perhaps the most fruitful of Ayer's life. He transformed the University College philosophy department into one of the best in the country, rivaling those of Cambridge and even Oxford. He edited several series of books, presided over various discussion groups, developed a friendship with his hero Russell, lectured around the world, and made lively contributions to literary journals and radio broadcasts. At the same time, he produced a series of influential papers—including "Statements About the Past," "Phenomenalism," "On What There Is," "On the Analysis of Moral Judgements," and "Can There Be a Private Language?" —and what was probably his most philosophically successful book, The Problem of Knowledge (1956).

In this work the great combative proclamations of Language, Truth, and Logic were replaced by a quieter treatment of Skepticism, in which Ayer presented the various theories of knowledge that have been propounded by philosophers as responses to a radical skeptic who argues for the existence of gaps between, on the one hand, the belief in an external world, in the existence of other minds, and in the reality of the past and, on the other, the evidence on which these beliefs are based. But whereas Ayer previously had in effect pursued a “reductionism” of all meaningful propositions to the sense-data by which they are verified, he now admitted that not everything can be translated into the language of the senses; instead, the constructions made on the basis of experience have their own inherent validity.

Ayer was metropolitan in his tastes, enjoying the company of writers, actors, and politicians as much as that of philosophers. He was especially close to Hugh Gaitskell (Gaitskell, Hugh), leader of the Labour Party until his early death in 1963, and later to the reforming Labour home secretary Roy Jenkins (Jenkins, Roy, Baron Jenkins of Hillhead). It was with some misgivings, then, that in 1959 Ayer returned to Oxford to become Wykeham Professor of Logic. As it was, his tenure there, until his retirement in 1978, proved extremely happy. Still fond of provoking and shocking others, Ayer moved to abolish the saying of grace before college dinner and publicly ridiculed the latest philosophical theories emanating from France. But he was a popular colleague and teacher and remained philosophically productive. In 1973 he published The Central Questions of Philosophy, in which he returned to familiar topics in the theory of knowledge and presented a commonsense conception of the world as a theory founded on the basis of sense-data.

The last decade of Ayer's life was troubled. In 1980 his first wife, Renee Lees, whom he had divorced in 1945, died, and one year later their daughter Valerie died suddenly of Hodgkin disease. In 1982 he divorced his second wife, the writer Dee Wells. His third wife, Vanessa Lawson (formerly married to Nigel Lawson, the chancellor of the Exchequer), died in 1984, leaving him bereft. Suffering from emphysema, he collapsed in 1988 and underwent a remarkable near-death experience, in which, as he later described, he seemed to encounter the “Master of the Universe” and his ministers for space and time. (His account was misunderstood by some critics as a recantation of his atheism.) Just before his real death in 1989, Ayer remarried Dee Wells and was united with his daughter born to the Hollywood gossip columnist Sheilah Graham. It was an end in keeping with his colourful, eventful private life.

Ayer once described himself as “Horatio to Russell's Hamlet.” But, while Russell was the superior logician, Ayer was arguably the more penetrating and imaginative epistemologist. Asked to assess his contribution at the end of his life, Ayer responded, “I suppose that I care more about having got something right in philosophy, if I have got anything right, than having written elegantly. Although I like that too.”