ARTHUR KOESTLER has led a remarkably full and varied life. Yet in most minds, I suspect, he is associated with a single issue, namely, anti-communism. Indeed, Koestler is perhaps the most famous of all ex-Communists. His fame dates from the publication in 1941 of his novel Darkness at Noon, a psychological and moral study of an Old Bolshevik during the Moscow Trials. Together with the powerful autobiographical essay on his years as a Communist in Richard Crossman's The God That Failed (1950), it is among the handful of writings that defined the intellectual climate of the postwar era.

The height of Koestler's influence came in 1950 with the founding of the Congress for Cultural Freedom. Largely his own inspiration, the congress brought together a number of European and American intellectuals with a view to enlightening "confused liberals" about Stalinist oppression. The beginnings of the congress corresponded with the two years Koestler spent in the United States, and perhaps for this reason he has tended to merge in the collective memory with the likes of Joseph McCarthy, Whittaker Chambers, and the young Richard Nixon. But Iain Hamilton's excellent biography makes clear that between the hysterical, self-serving anti-communism of the American right and Koestler's principled, almost metaphysical, opposition to Soviet tyranny, there is only limited correspondence.

A Hungarian Jew, with continental intellectual sympathies, Koestler was, and remains, essentially a man of the left. He has never shown any interest in defending the "free" enterprise system. Indeed, the culture of American capitalism is apparently as distasteful to him as it was to so unambiguously radical a thinker as Herbert Marcuse. Spiritually he is much more at home in the company of Jean-Paul Sartre and Simone de Beauvoir (with whom he maintained relatively cordial and usually respectful relations in the late 1940s) than with William F. Buckley or Daniel Bell. His hatred of communism rests on convictions both narrower and more disinterested than those of the typical American anti-communist: Soviet communism is evil, for Koestler, because it tells lies and destroys life. Opposing it does not require us to think well of American society or American foreign policy, only to recognize that on one side is "absolute tyranny," on the other "relative freedom" -- and "relative" can encompass a multitude of sins. The important thing is not to confuse the Moscow Purges with the Hollywood Purges. Because he defines the issue in such austere terms, Koestler avoids any suspicion of bad faith. Appropriately, he was not a bit embarrassed when, in the 1970s, scholars discovered that the Congress for Cultural Freedom had been secretly funded by the C.I.A. Since his own motives were pure, the worst he would say of the arrangement is that it had been clumsy.

The difference between American and Central European intellectual manners is also reflected in Koestler's other political obsessions, none of which has a place in the repertory of the American right. The earliest of these was Zionism. In the 1920s he became a follower of Vladimir Jabotinsky, urging the speedy establishment of a Jewish state on both sides of the Jordan. Later he moderated into an advocate of "partitionism," arguing that Palestine should be divided between the Jews and the Arabs. Yet even in 1948 he remained sympathetic to the most extreme Zionist groups, in particular Menachem Begin's terrorist militia, Irgun Zvai Leumi.

No less anomalous from the perspective of American conservatism has been his passionate opposition to capital punishment, a subject on which he wrote extensively in the 1950s and 1960s. With Victor Gollancz he organized a national campaign for the abolition of capital punishment in Britain, and he also began a trust fund to award prizes to prisoners with unusual artistic ability. As his biographer notes, Koestler himself knew the horrors of prison life only too well, having been jailed as a spy by Franco for three months in 1937, and then detained as an undesirable alien by Vichy for a slightly longer period in 1939, and by the British for a month and a half in 1940.

Although he will be remembered mainly for his political writings, Koestler had hoped to make his reputation as a historian and philosopher of science, to which he devoted his longest and most ambitious books. Through his many writings on science runs a single theme: scientific discovery, he maintains, is a product not of rationalism and empiricism, but of the same unconscious processes that give rise to works of art. He is, accordingly, a fierce critic of the mechanistic and progressivist conception of science inherited from the 19th century. Despite their considerable bulk, however, his scientific writings have not earned him the regard he anticipated. Too often his synthetic ambition seems to have exceeded his capacity for conceptual discipline, lending his arguments an elusive and slightly bloated quality. More significantly, he has been outflanked by the American historian of science, Thomas Kuhn, whose book The Structure of Scientific Revolutions advances a number of the same propositions, but does so in a more modest, accessible, and -- it must be admitted -- rigorous fashion. Kuhn's book has been the most influential writing on the history of science published in the last two decades, while Koestler's works appear to have left little impression.

In recent years Koestler has grown increasingly interested in parapsychology and drugs. Abetted by Timothy Leary he experimented tentatively with magic mushrooms, and although he dismissed Leary's more extravagant claims, he nonetheless scandalizeddmany of his admirers by proposing a pharmaceutical solution to the human predicament: the answer to human aggression, he suggested, might lie in some sort of "Peace Pill."

I am inclined to agree with Iain Hamilton that this slightly daffy peroration of Koestler's career should not be used as ammunition against his earlier achievements, even though the continuities are certainly there to be identified. Quite sensibly, Hamilton devotes most of his book to the political and scientific writings of the middle decades. He also offers a vivid and believable portrait of Koestler as an obsessive, contentious, and authoritarian personality, but at the same time one capable of inspiring great loyalty and affection. Until he settled in Britain in 1952, he led a compulsively peripatetic existence, which doubtless has given his biographer (not to mention his publisher) many a nightmare: six months in one country appears to have been his upper limit. Hamilton's narrative, as a result, moves restlessly across the map of Europe, the United States, and the Middle East. The cast of characters is no less daunting, incorporating most of the intellectual celebrities of the post-war era. There is also a good deal of womanizing, boozing, squabbling, and brawling--even one memorable night when Koestler threw a glass at Sartre and a punch at Camus. Through it all, Hamilton is a comfortable, sympathetic, but not uncritical guide.