FOR PEOPLE of a certain age, Arthur Koestler was among the most authentic witnesses to the century's temptations and terrors. Born in Hungary as the old empire disintegrated, he grew up in Vienna as the postwar order disintegrated too. Seeking a safe shore, he turned first to Zionism of the Jabotinsky stripe, in Palestine, but then moved back to the larger stage, Europe, and on to an ostensibly more universal saving creed, communism. In Spain during its civil war, in Russia during the Stalin purges, in France under the German occupation, he saw up close, variously as an engaged observer and a prisoner and refugee, the harshest stuff of his times.

From this exposure came dozens of books of which "Darkness at Noon," published in 1940 after Mr. Koestler had broken with the Communist Party, remains preeminent and--more important-- widely read. In this classic portrayal of the communist mind and the Soviet system, a still-faithful Old Bolshevik confesses to crimes he did not commit. Ten years later, Mr. Koestler made another influential and enduring contribution to the literature and cause of anti-communism--in "The God That Failed," a volume of statements by ex-communists on what had gone wrong. His testimony struck a note of intellectual penetration and personal disinterest that was to become increasingly rare as the Cold War went on.

In the second half of his career, Mr. Koestler made an effort to leave the great political events and ideas that he had earlier devoured, and that had nearly devoured him. But the same intensity and restlessness stayed in play. As one who had moved from a quest for political certainties to the study of the means of control offered by modern science, it was hardly surprising that death came to him as it did. He belonged to an organization devoted to "the right to die with dignity." A failing 77, he was found this week in his London living room, with his wife, both of them suicides.