same year succeeded Julian Huxley as president of the British Humanist Association, a post he held until 1970. In 1968 he edited The Humanist Outlook, a collection of essays on the meaning of humanism. He was one of the signers of the Humanist Manifesto. Works Ayer is best known for popularising the verification principle, in particular through his presentation of it in Language, Truth, and Logic. The principle was at the time at the heart of the debates of the so-called Vienna Circle, which Ayer had visited as a young guest. Others, including the circle's leading light, Moritz Schlick, were already writing papers on the issue. Ayer's formulation was that a sentence can be meaningful only if it has verifiable empirical import; otherwise, it is either "analytical" if tautologous or "metaphysical" (i.e. meaningless, or "literally senseless"). He started to work on the book at the age of 23 and it was published when he was 26. Ayer's philosophical ideas were deeply influenced by those of the Vienna Circle and David Hume. His clear, vibrant and polemical exposition of them makes Language, Truth and Logic essential reading on the tenets of logical empiricism; the book is regarded as a classic of 20th-century analytic philosophy and is widely read in philosophy courses around the world. In it, Ayer also proposes that the distinction between a conscious man and an unconscious machine resolves itself into a distinction between "different types of perceptible behaviour", an argument that anticipates the Turing test published in 1950 to test a machine's capability to demonstrate intelligence. Ayer wrote two books on the philosopher Bertrand Russell, Russell and Moore: The Analytic Heritage (1971) and Russell (1972). He also wrote an introductory book on the philosophy of David Hume and a short biography of Voltaire. Ayer was a strong critic of the German philosopher Martin Heidegger. As a logical positivist, Ayer was in conflict with Heidegger's vast, overarching theories of existence. Ayer considered them completely unverifiable through empirical demonstration and logical analysis, and this sort of philosophy an unfortunate strain in modern thought. He considered Heidegger the worst example of such philosophy, which Ayer believed entirely useless. In Philosophy in the Twentieth Century, Ayer accuses Heidegger of "surprising ignorance" or "unscrupulous distortion" and "what can fairly be described as charlatanism." In 1972–73, Ayer gave the Gifford Lectures at the University of St Andrews, later published as The Central Questions of