From one point of view, the fact that the twentieth century's most important philosopher is also its most notorious is of no importance. As the famous scholar of medieval philosophy, Etienne Gilson, once observed, everything in the history of philosophy that can be traced to history is irrelevant to philosophy. The sometime Nazi, Martin Heidegger, may be the only Western philosopher better known for his person than his doctrine, but that cannot touch his achievement in permanently altering the landscape of philosophical thought.

And yet, from another point of view, the moral failings of a philosopher not only outweigh his philosophy, but wreck it. We may accept that a great mathematician or even a great artist can also be immoral, but we expect something more from thinkers whose model is Socrates. The accumulation of Heidegger's moral blind sports rightly makes readers suspect the philosophy that allowed them.

Ever since the end of World War II, the estimation of Heidegger has swung between these two points of view. Immediately after the war, he was widely denounced as Hitler's theorist and apologist, and the allied censors in occupied Germany refused him permission to teach. But in the early 1950s, his former student Hannah Arendt published an essay that both introduced to America the word "existentialism" and began the restoration of Heidegger's reputation. When he died in 1976, newspaper obituaries still referred to him as the "noted Nazi philosopher." But among intellectuals by the late 1970s, he seemed rivaled only by Ludwig Wittgenstein as the greatest philosopher of the century.

Within a few years, however, the accepted view of Heidegger quickly began to change. In 1987, Victor Farias's Heidegger and Nazism was published in Europe, documenting the massive extent of the philosopher's involvement with Hitler's government. Just as the "Heidegger controversy" seemed to be dying down, Elzbieta Ettinger fanned it again in 1995 with a study of Heidegger's affair with and wretched treatment of Arendt, who seems to have remained in love with the philosopher until he died. Over the last three years, at least a dozen books have appeared in America denouncing Heidegger and hunting through his philosophical writings for the flaws that produced and justified his behavior.

It is Rudiger Safranski's great merit that he endeavors in his new biography Martin Heidegger: Between Good and Evil to present unflinchingly both these ways of viewing Heidegger. A great deal of the contemporary reaction against the philosopher seems motivated by current standards of political correctness in American academia. But there is no way around the fact, for example, that Heidegger's inflated view of his own place in the history of philosophy is one of the things that led him to great the rise of Hitler as a new chapter in the history of Being. His speculations before the Nazis may seem confined almost entirely to the abstractions of metaphysics, but Safranski points to a passage in the classic 1927 Being and Time where Heidegger avers that only a "hero" can fill the cavity of Nothingness lurking in the human soul.

The passage, as Safranski notes, occurs in a book little known for its call to historical action. Nevertheless, Heidegger became convinced by the instant success of Being and Time that he had a more crucial role to play in the history of the West than merely providing a philosophical diagnostics. And so when Hitler came to power, Heidegger duped himself into believing that he could shape National Socialism into his own eccentric philosophical mold.

Within months of Hitler's accession, Heidegger accepted the rectorship of Freiburg University and immediately began running the school according to the Fuhrer principle: Jewish professors were dismissed, the Faculty Senate was abolished, and students were led off to what Heidegger rather grimly called a Wissenschaftslager -- "Scholarship Boot Camp" -- where the "sterile preoccupation with the ideologies of Christianity and positivist fact-mongering" were to be overcome with pep talks from the Master himself. "They departed from the university in close marching order," Safranski writes. "Heidegger had selected a small circle of associate professors and students [with] these stage directions: 'The company will proceed on foot and SA or SS uniforms will be worn.'"

One result of this absurd outing was to make the man look silly in the eyes of both faculty and students, but Heidegger seems to have gained from it a picture of himself as the thinker who brought philosophy into living history: the great man in "the vanguard of the revolutionary students," who "in knee-breeches and with an open-necked shirt" stands "against the stodginess underneath the academic gowns."

Such self-absorption is evident not merely in his Nazi period. Safranski considers the possibility that Heidegger abandoned his native Catholicism precisely because he had been so favored by the Church. Born into a lower-middle-class household in a small Swabian village in 1889, he could never have afforded the education he received except at church-supported schools; when poor health forced his departure from the seminary, he presented himself as a promising young Catholic philosopher in order to qualify for educational financing. But -- at least according to Safranski in one of his more speculative moods -- the generosity of his benefactors made him seethe with resentment against the system that had supported him.

After the accounts of these and many other glaring episodes in Safranski's unsparing biography, the temptation becomes nearly irresistible to dismiss the doctrine because of the man. But Safranski refuses to allow the reader to escape so easily. Without ever bluntly calling such a dismissal vulgar or lazy, he makes clear that Heidegger has thrown up a mountain of ontological reflection that cannot be ignored.

In Martin Heidegger, Safranski reveals a remarkable talent for presenting complex philosophical ideas in an accessible and pithy way. He is clear without distorting the complexity of the issues, and his chapter on Being and Time is the best essay-length introduction to that difficult work. His style, however, may strike some readers as wooden. He calls Heidegger's prose "laconic but cumbersome," though his own is perhaps best described as "terse but choppy" -- and the translation by Ewald Osers, while serviceable, seems to have little feel for English idiom or cadence.

The somber Teutonism of the text and translation, however, end up serving the point of the biography. Martin Heidegger makes no attempt at exoneration: Safranski shows clearly, for example, that Heidegger resigned his rectorship not because of disaffection from the Nazis (as he later pretended), but because of his frustration at the lack of zeal and purity in the revolutionary movement. But the book is at the same time free of the kind of moralism that replaces nuance with indignation. In Safranski's biography, Heidegger emerges as a philosophers' version of Rain Man: the idiot-savant who transformed twentieth-century philosophy and yet could lead -- in his William Tell lederhosen -- a group of overage Nazi Boy Scouts to his hut in the Black Forest and think he was thereby inaugurating a new chapter in the history of Being.

Not even the most despicable of Heidegger's actions ever dissuaded those who knew him of his genius. Nearly every student who took his seminars -- from Hannah Arendt to Karl Rahner and Hans-Georg Gadamer -- speaks of Heidegger's astonishing ability to make philosophers come alive in the seminar room, as though he were conducting a seance with the departed spirits of philosophy. Moreover, as Safranski points out, there is not a whisper of racism in Heidegger -- something noted by the Nazis themselves when they prevented him from joining the official German delegation to the 1937 Descartes Conference in Paris.

None of this exonerates either the philosopher or his philosophy. But it does point toward a way to judge them, both together and separately.

Safranski is no apologist for Heidegger, but he sets out the issues and positions fairly, sympathetically, and succinctly. And he shows how genuinely disputable Heidegger's thought remains -- its philosophical profundity neither entirely untouched nor entirely ruined by the man's failings. Perhaps the most exciting chapter in Martin Heidegger is Safranski's account of the public debate between Heidegger and the famous neo-Kantian and Enlightenment-enthusiast Ernst Cassirer, who shortly after their encounter in the spring of 1929 was elected Rector of Hamburg University, the first Jewish rector of a German University.

As it happens, their stylized debate took place in the same Swiss mountain town, Davos, where Thomas Mann had placed the fictional debate between the humanist Settembrini and the Jesuit Naphta in his 1924 novel The Magic Mountain, a coincidence the author uses to great effect. Here on the same dais were Cassirer, the grand seigneur of political humanism and cultural idealism, and Heidegger, the spearhead and enfant terrible of all that was new and revolutionary in German thought.

It is a mark of the balance of his remarkable biography that Safranski can conclude both that Cassirer was right and that Heidegger was deeper -- for it was Heidegger's achievement to see how culture can anesthetize man by cosseting him in cozy everydayness and aesthetic indulgence. Heidegger saw his task not to humanize the barbarian, as Cassirer wanted to do, but "to throw man out of his lazy habit of using works of the spirit to hide from the hardness of his fate."

One finishes this new biography with a rare sense of a balance achieved -- a recognition of Heidegger's genius coupled with a commensurate dismay that his genius did not prevent and in some cases encouraged his personal failings. Safranski has done Heidegger a great service, but only because he has performed an even greater service to the truth.

Edward T. Oakes S.J. teaches in the religious studies department of Regis University in Denver, Colorado.