The mole, the penetration agent in particular, does not merely betray; he stays. He doesn't just commit a single treacherous act and run; his entire being, every smile, every word he exchanges, is an intimate violation (an almost sexual penetration) of all those around him. All his friendships, his relationships, his marriages become elaborate lies requiring unceasing vigilance to maintain, lies in a play-within-a-play only he can follow. He is not merely the supreme spy; he is above all the supreme actor. If, as le Carre once wrote, "Espionage is the secret theater of our society," Kim Philby is its Olivier.

And, like only the very best actors, Philby didn't merely hold up a mirror to human nature. He revealed dark shapes beneath the surface only dimly glimpsed before, if at all -- depths of duplicity, subzero degrees of cold-bloodedness that may not even have been there until Philby plumbed them. Once in an interview on another subject, the essayist George Steiner made the provocative suggestion to me that the nightmare world of the death camps might not have been realizable had not Kafka's imagination first embodied their possibility in his fiction. I have a similar feeling that the Age of Paranoia we've lived in for the last half century -- the plague of suspicion, distrust, disinformation, conspiracy consciousness that has emanated like gamma radiation from intelligence agencies East and West, the pervasive feeling of unfathomable deceit that has destabilized our confidence in the knowability of history -- is the true legacy of Kim Philby.

Philby pushed the permutations of doubleness -- double identities, double meanings and double crosses -- into triply complex territory, into the bewilderment of mirrors we're still lost within. He's the high priest of the Age of Paranoia, the original disinformation virus, and we're still only beginning to learn how much of the secret history of the century bears Philbian fingerprints.

Unlike the spy scandals of the 40's and 50's, the Philby case has been a slow-motion series of revelatory detonations stretched out over decades. One reason the truth has been so slow in emerging is that it's just so embarrassing. Even before James Bond, the spymasters of the British Secret Service enjoyed a worldwide reputation for infinite subtlety, invincibility and aristocratic elan. Philby made them seem like bumbling fools who were so blinded by class prejudice they couldn't imagine that a man from all the right schools and all the right clubs could betray his blue-blood legacy.

Indeed, the deeply chagrined British Government clamped such a tight lid on the Philby case that it took nearly five years after he defected to Moscow in 1963 for the most embarrassing truth to come out (in a ground-breaking Sunday Times of London investigative series): that Philby was no ordinary midlevel spy; that, in fact, he'd been one small step away from being named to one of the most powerful posts in the Western world at the height of the cold war -- Chief of the British Secret Service. (Although British officialdom pooh-poohed this assertion at the time, it was confirmed to me in London this spring by Sir Patrick Reilly, the former head of the Joint Intelligence Committee, the board of spy mandarins that oversees the selection of "C," the chief of the British Secret Intelligence Service.)

Philby was no less a nemesis of the American spy establishment. In the final act of his active duty career in the West, before the spotlight of suspicion fell on him in 1951, Philby was stationed in Washington, where, as chief British liaison with American intelligence agencies, he charmed the C.I.A.'s deepest secrets out of his principal contact, James Jesus Angleton, the man who would go on to become legendary as the C.I.A.'s chief mole hunter. This shattering betrayal left behind a destructive legacy of distrust and paranoia in Washington -- principally in Angleton's mind -- reverberations of which would come to plunge the C.I.A. into civil war for decades afterward. And in an incredible final act that closed the circle of deceit, in what may have been his last operational mission, Philby indirectly collaborated with Aldrich Ames in solving a high-level mole case for the K.G.B.

But these spy dramas only begin to capture the extent of Philby's role in the secret history of our century, the extent to which he was far more than a cold war spy -- he was a secret shaper of the very landscape of the cold war.