The life of Hungarian-born writer Arthur Koestler, and his reputation, have revolved around the themes of science and communism; and in each area his contribution has been controversial. Former Spectator editor Hamilton greatly admires Koestler, and has had access to some of his diaries and private papers; but his spare, uncritical account of Koestler's life, though interesting as the first full biography, does little to illuminate those controversies. Koestler's early background in science, coupled with his considerable writing skill, started him on a career in science journalism (after a fling with Zionism--under the spell of right-winger Vladimir Jabotinsky--and a reportorial stint in Palestine). But following his appointment as science editor of a Berlin newspaper (he was also foreign editor), Koestler moved, in 1930, toward the Communist Party--a move that eventually led to his dismissal from the paper and his first trip to the USSR. Souring on the Russian Revolution (after a short spell, also, in a Spanish Nationalist jail), he wrote the novel Darkness at Noon, on the Stalinist purges--which made his name and established him as the archetypal ex-communist anti-communist. In that role he was a co-founder, in 1950, of the Congress for Cultural Freedom. (Hamilton, following Koestler's lead, matter-of-factly notes that the Western powers were ""bamboozled"" by Stalin at Yalta; and comments that Koestler was ""smeared"" as a McCarthyite by deluded champions of Alger Hiss.) Koestler then began to shift back toward his earlier scientific interests and to develop a more far-ranging criticism of rationalism. He now saw two threats to civilization: communist subversion and unchecked scientific reasoning. In a series of works beginning in 1954, he started to delve into the history of science and to evolve a theory of irrational intuition underlying the scientific discoveries of creative geniuses like Kepler. With publication of The Sleepwalkers, and other works on this theme, he found himself assaulted by the academy as a dilettante and an interloper. Hamilton, claiming lack of expertise, passes no judgment on the value of Koestler's work--leaving the impression, however, that Koestler was rejected as outside the scholarly pale. In Koestler's subsequent, more mystical writings (on Eastern religions, mind-expanding drugs, and ESP), he Finds little to praise; so he merely catalogues them. Koestler has gone near, if not off, the deep end. To the extent that the man emerges, he appears obsessed with writing, prone to tantrums when his writing is not going well, given to drink a good deal, and distinctly self-assured. This is for readers with a knowledge of Koestler's writings (even Darkness is not explicated) and, given the choppy narrative, a preexisting interest in his contribution--about which they will then have to decide for themselves.