Alfred Jules Ayer was born in north-west London on 29th October 1930, the son of Jules Ayer, a Swiss financier, and Reine Citroen, who came from a Dutch Jewish Family associated with the French car manufacturer. He was educated at Ascham St Vincent School in Eastbourne and at Eton College, and in 1929 went up to Oxford with an open classics scholarship at Christ Church College. After graduating in literae humaniores in 1932, Ayer spent the winter of 1932-1933 in Vienna, where, on the advice of his Oxford tutor Gilbert Ryle who had introduced him to the ideas of Bertrand Russell and Ludwig Wittgenstein, he attended lectures by Moritz Schlick and meetings of the Vienna circle.

In 1933, Ayer took up a lectureship at Christ Church College, and in 1936, he published his first book, Language, Truth and Logic, in which he put forward the major thesis of logical positivism and which is widely regarded as a classic of 20th century analytic philosophy. By the time of the publication of his second book, The Foundations of Empirical Knowledge (1940), in which he formulated his sense-data theory, Ayer was in the army, where he mainly served in intelligence. He worked for the British Security Co-ordination in New York, before he joined a French section of the Special Operations Executive (SOE). Promoted captain in September 1943, he ended the war attached to the British Embassy in Paris.

In 1945, Ayer briefly returned to Oxford as a philosophy tutor at Wadham College, but in 1946 he moved to London, after having obtained the Grote chair of the philosophy of mind and logic at University Collge London. He published Philosophical Essays (1954) and The Problem of Knowledge (1956), travelled widely on lecture tours, and started to appear on the radio and on television. He was involved in many BBC Third Programme broadcasts, including a famous debate with the Jesuit priest Frederick Copleston on the existence of God, and later became a regular performer on the BBC television programme ‘The Brains Trust’.

Having accepted the Wykeham chair of logic at New College, Ayer returned to Oxford in 1959. In addition to teaching and writing, he continued to travel on lecture tours and for conferences and visits, and was involved in many political activities from supporting the Labour Party (and later the Social Democratic Party) to campaigning against the British involvement in Vietnam. He was closely associated with the British humanist movement, becoming the president of the British Humanist Association and of the Independent Adoption Society in 1965, and in 1968 editing The Humanist Outlook. He retired from the Wykeham chair in 1978, but remained a prolific writer and international scholar. He became a fellow of Wolfson College, and during the late 1970s and 1980s lectured extensively in North America, including serving as a visiting professor at Bard College, New York.

Ayer was married four times; from 1932 to 1941 to Renée Lees (1909-1980), with whom he had two children, Valerie (1937-1981) and Julian (1939-2004); from 1960 to 1983 to Alberta Constance (Dee) Wells (1925-2003), née Chapman, with whom he had a son, Nicholas (b. 1963); from 1983-1985 to Vanessa Addison (d. 1985), former wife of Nigel Lawson MP; and, from April 1989 to his death in June that year, again to Dee Wells. He also had a daughter, Wendy, with Hollywood columnist Sheila Graham Westbrook.

Alfred Jules Ayer is usually cited as A.J. Ayer, and was known as ‘Freddie’ to his friends and colleagues. He was well connected, both in intellectual circles and in ‘high society, and enjoyed socialising at clubs in London and New York, and at college diners in Oxford. Having played rugby for Eton, sports remained a lifelong passion and, apart from being a noted cricketer, he was a keen supporter of Tottenham Hotspurs football team.

Ayer was made a fellow of the British Academy (1952) and a foreign honorary member of the American Academy of Arts and Sciences (1963), was knighted in 1970 and became a chevalier of the Legion d’honneur in 1977. He received honorary degrees from Brussels (1962), East Anglia (1972), London (1978), Trent (1980), Bard (1983) and Durham (1986), and became an honorary fellow University College London in 1978 and of New College, Oxford, in 1980.

Ayer died at University College Hospital, London, on 27 June 1989.