Books: Martin Heidegger: between good and evil by Rudiger Safranski Harvard University Press, pounds 23.50: `Have you seen Hitler's hands?'

Heidegger was a Nazi. He was also the greatest thinker of our century." This statement comes not from one of Hitler's acolytes but from Emmanuel Levinas, a Jewish philosopher who survived the Holocaust and pioneered the primacy of ethical thinking in our time. How to resolve this paradox?

Rudiger Safranski's lively biography attempts a response. Neither apologist nor accuser, Safranski treads a delicate winding path through the Black Forest of the modern German mind. The result is impressively judicious, offering us a privileged glance into that nation's intellectual unconscious.

One cannot understand Martin Heidegger, who lived from 1899 to 1976, without grasping an intricate pattern of ideas crisscrossing Europe, taking in figures such as Kierkegaard, Sartre, Camus, Marx, Freud and Wittgenstein. So Heidegger emerges not as some exotic exception but as a thinker who harnessed the philosophical energies of a generation eager for a spiritual revolution that would enable history to begin again.

Some saw this promise in Stalin, others in Mussolini or Mao. Heidegger, for a while at least, saw it in Hitler. When asked by a colleague how he - the great philosopher opposed to racism, mass hysteria and anti-Semitism - could have made such a mistake about the reality of Nazism, Heidegger replied: "Have you not seen Hitler's hands?"

While never enamoured of the crudities of Nazi ideology, Heidegger was fascinated by the prospect of a practical revolution that would challenge the nihilism of modernity. Provided, of course, it was prepared to take guidance from a great philosophical mind. Heidegger saw himself, during his short tenure as Rector of Freiburg University in 1933, as the intellectual heir to the emerging National Socialist revolution. He would be to the new Germany what Hegel might have been to Prussia, Plato to Syracuse, or Marx to the Communist state.

His dream of the philosopher-king failed. Hitler's lackeys threw his proposals for a Ministry of University Reform back in his face and told him to go back to his hermit hut on the Todnauberg mountain. Which he did, with his tail between his legs, for the rest of his days. There he would write most of his remaining works in altitudinous retreat, when not teaching in Freiburg or receiving visits from the leading thinkers of his time.

The fact that most of Heidegger's intellectual admirers after the war were Jews lucky to escape the holocaust (Hannah Arendt, Paul Celan), left- wingers such as Sartre and Marcuse, or Resistance leaders such as the poet Rene Char, is a token of just how puzzling the philosopher's brief conversion to Nazism in 1933 actually was. His friends understood that, even though he was one of the most profound thinkers about the modern destiny of Being, he was incapable of sound judgement at a moral level. Entranced with the big question of Being, he forgot about the needs of ordinary human beings. Enthralled with mountain peaks, he ignored life down in the valleys.

Thus he could, for example, argue that "the motorised food industry is essentially the same as the manufacture of corpses and gas chambers". Safranski explains the context of such a statement - the domination of every aspect of modern life and death by technology, which is itself a symptom of our forgetfulness of Being. What still shocks is that this very attentiveness to the dark destiny of the West goes hand in hand with an extraordinary moral insensitivity: the equation of agriculture with Auschwitz.

Heidegger may have come close to some kind of reckoning when he confessed after the war: "Nietzsche undid me!" The attempt to assume the role of superman going "beyond good and evil" in order to think and live dangerously leads to moral blindness. That such blindness could in this case partner some of the best philosophical work of this century is a puzzle indeed. Great minds are not always great moral beings. We like to think that good writing makes for good living. But it ain't necessarily so.

Safranksi's biography is brisk, lucid and illuminating. The manner in which he weaves Heidegger's thinking into the intrigues of his life makes for fascinating reading - nowhere more so than in Heidegger's relationship with Hannah Arendt, his secret lover in the 1920s and his loyal advocate after the war. His book also gives an excellent account of Heidegger's "existential" ideas, highlighting the inimitable charisma surrounding both his writing and person. This is a towering biography of a giant intellect.