After Bertrand Russell, A J Ayer was the best-known British philosopher of the 20th century. He was one of those publicity-hungry men, like A J P Taylor in history or, in another arena, Alfred Hitchcock and Orson Welles, who had a public persona utterly distinct from his serious work as a professional. As in all such cases, the perception of the personal feeds back into the estimation of the serious work: such people are all underestimated because televisual familiarity tends to breed contempt. Ayer was perhaps less worried about this than any of the others mentioned. Far from believing in philosopher kings, he thought, unlike, say, Wittgenstein, that philosophy and life were things apart. Life was for living with all the gusto that a strenuous hedonist could muster; philosophy, a technical matter of conceptual analysis, was for the study and the lecture hall.
Just as Ernest Jones was the apostle to the British from Freud's Vienna circle, so Ayer played a similar role on behalf of the Vienna circle of logical positivism. His first book, Language, Truth and Logic, published when he was 26, grossly oversimplified all philosophical problems but was a succes de scandale in its dismissal of metaphysics and theology as "literally nonsense". Always impatient with Continental metaphysics, Ayer showed some sympathy for the questions raised by existentialism but regarded Heidegger as a charlatan. But Ayer misunderstood Heidegger and Sartre. He grappled with their notion of "Nothingness" (Le Neant) as a linguistic confusion, in the style of Lewis Carroll ("Nobody ran faster than me." "That's impossible, for if so, he would have got here before you."). But Heidegger made it clear that Le Neant, like evil in traditional thought, was a living force; Ayer's "refutation" was therefore as absurd as the official stance of Thomistic philosophy, that evil is privatio boni - the mere absence of good.
Yet although Ayer underrated non-positivistic philosophies, he always believed that philosophy had to be the handmaiden of science, that scientific knowledge was the paradigm form of knowledge. He struggled hard to master the mathematical knowledge needed to be a philosopher of the stature of Russell, Whitehead or Quine, gave up and frankly admitted he was not in their class. But he was light years ahead of the so-called Oxford "ordinary language" philosophers, who, instead of admitting that they lacked the intellectual training to be "real" philosophers, disingenuously pretended that a knowledge of the dictionary and lexicography fitted them for the task of conceptual analysis. Ayer honestly conceded that he would be happy to be remembered as a footnote to Russell, but this alarmed the Oxford school, who, having earlier tried to blight Ayer's career, later tried to build him up as "one of us".
Russell was always the greatest influence in Ayer's intellectual life: he conceded that he had known only three really great men, Russell, Einstein and e e cummings, with whom Ayer had a lifelong if unlikely friendship. Ayer's judgement on greatness deserves to be taken seriously, since he met and conversed with an exceptionally wide range of people - Bobby Kennedy, Eartha Kitt, Clive Donner, Orwell, Giacommetti, V S Pritchett, Camus (though not Sartre, who refused to meet him and remarked: "Ayer est un con.") But Ayer was cold and often inhuman in his dealings with others. He suffered from hypertrophy of intellect at the expense of feeling and intuition, to the extent that Rogers speaks of autism - Ayer was one of those individuals for whom people do not exist unless they are present here and now, an unfortunate by-product of his "sense-data" approach to the world. Other people interested him very little, yet he claimed that the philosophical problem that most interested him was "other minds".
Ayer's coldness was most apparent in his dealings with women. Where the Oxford school of linguistic philosophy tended to be sexual oddities - Wittgenstein a tortured homosexual, Gilbert Ryle a lifelong virgin, John L Austin a male hausfrau - Ayer was a womaniser in the Simenon or Victor Hugo class. He often seemed to run half a dozen affairs simultaneously and was so insouciant that he wrote identical love letters to two of his mistresses; unfortunately, unknown to Ayer, they were friends and literally compared notes.
Ben Rogers is very convincing on the philandering. He sees it partly as Ayer's determination, as an atheist, to extract the last ounce of pleasure this life had to offer; partly a consequence of his louche lounge-lizard side (he was a superb dancer, with the samba his speciality); and partly to do with his love of conquest: intensely competitive, Ayer had to win or be first at everything, whether solving the Times crossword puzzle, making a grand slam in bridge or out-pointing an opponent in a debate.
A J Ayer lived a fascinating life and in Rogers he has found an ideal biographer. Unlike most dons, Ayer could have succeeded in a number of other careers, where he performed with distinction, including the army, the foreign service and journalism. Though not in the front rank of 20th-century philosophers, alongside Russell, Whitehead, Dewey and Heidegger, and not quite in the reserve booths occupied by Quine and C I Lewis, he was way ahead of any British thinker except Russell. In all kinds of ways he seemed always to tread in Russell's footsteps. When Russell in the 1960s phoned Muhammad Ali to congratulate him on his refusal to serve in the Vietnam war, a euphoric Ali replied: "You're not as dumb as you look."
Ayers' fistic encounter was with Mike Tyson. When Tyson was harassing a woman in a New York nightclub, the audacious Ayer intervened. Tyson yelled: "Do you know who the fuck I am? I'm the heavyweight champion of the world."
Ayer replied: "And I am the former Wykeham professor of logic. We are both pre-eminent in our field." Can one imagine any of the etiolated and desiccated spirits of Oxford - Isaiah Berlin, say - getting involved in such a fracas? It is that kind of thing that makes one respond warmly to Ayer, and is one of many outstanding anecdotes that enliven Rogers' excellent book, masterly in its exposition of the philosophy as much as in its analysis of the life.
Frank McLynn's most recent book is "1066" (Jonathan Cape, £18.99)