Stranger from Abroad, By Daniel Maier-Katkin

Martin Heidegger and Hannah Arendt have many enemies, Heidegger because of his Nazi flirtation and Arendt as a critic of the Jews. We have known for some time, from their letters, that Heidegger, who reinvented philosophy in the 1920s, and Hannah, his star pupil who went on to become a leading political philosopher in the US, were briefly lovers in her teens. The tendency, which Daniel Maier-Katkin, an American human-rights professor magnificently resists, has been to use their early relationship further to indict what came later. In fact, their correspondence tells an exemplary tale of friendship and forgiveness in which the banality of human nature is redeemed.

Two tags commonly get attached to Heidegger's name. First, he's difficult. Here one can't really complain. A man who insisted philosophers return with primal astonishment to Being, after centuries of working within Christian rationalist doctrine, needed a new language for a new way of thinking. The obfuscation wasn't simply wilful. Second, he was a Nazi philosopher. In truth, he was far too complex a thinker, and his own man. He was, though, an opportunist and a nationalist, and for his Party membership, his spell as Freiburg university rector, and his collusion in the eviction of Jewish professors, he is blameworthy. As Hannah later told her second husband Heinrich Blücher, Heidegger was a mixture of "genuineness, mendacity and cowardice".

In Stranger from Abroad, she becomes a key to understanding him. Arendt was 17 years younger than her professor-lover. Born in 1906, she grew up in Königsberg, east Prussia, at a time when the German Jews were the most emancipated in Jewish history. Arendt was one of several young Jewish acolytes around Heidegger. She fled Germany in 1933 after being briefly arrested, joined other émigrés in Paris, where she met Blücher, and after the Nazi invasion of France in 1940 reached America, where she made her career.

Arendt was positive about her Jewish heritage, but in a detached, secular way which has almost faded from view today. When in 1963, as part of her desire to understand her people and her time, she later famously examined the "banality" of the evil done by the arch-logistician of the Final Solution Adolf Eichmann, she decided the Nazis were not the only perpetrators. Jewish leaders were also to blame.

Her Eichmann in Jerusalem was a complicated argument, based on a mixture of fact and sociological and moral observation raised to the level of philosophy. It came out at that trial, for instance, that Jewish leaders in Hungary had bargained with the Nazis to save an elite few, including themselves, while sending the innumerable rest to the gas. Arendt as both reporter and philosopher had to account for that.

She took the view, already a decade before the war, that Jews had two choices in any society: to flatter their way into the establishment or to accept their status as pariah. In pre-war Europe, as Maier-Katkin puts it, she thought "ordinary people became anti-Semitic because the Jewish leadership invariably aligned their communities with whoever was in power". Better then for Jews to see themselves positively as pariahs, or outsiders, and retain their self-respect.

The Eichmann trial focused on the catatrophe that befell the Jews. Arendt saw it more as a study in the nature of evil. Devastatingly, she concluded that evil was not radically different from good, but potentially always with us, whatever our origins. The proposition that Jews could also do evil, and that what had happened was not uniquely German, set her on a collision course with convenient post-war platitudes.

The hatred she endured in 1960s America for her independent stance on Israel is staggering: 60 years on, anyone who despairs of the current Middle East impasse would do well to read Maier-Katkin's beautifully sober account of how Arendt rejected an ethnically-based Jewish homeland and foresaw an inevitable and unwinnable conflict with the Palestinian Arabs who were, as she insisted on the firmest of post-Holocaust principles, also human beings.

Meanwhile, in 1950, already an eminent academic, she had been asked to spearhead the return to Europe of Jewish books the Nazis had confiscated. A huge collection had ended up at the Library of Congress. The occasion prompted her to visit Heidegger in his home town of Freiburg after 18 years of mutual silence. Banned from teaching for five years, but also treated as a scapegoat, Martin had suffered a nervous breakdown and was distraught. Now Hannah came and forgave him. Meier-Katkin gives a fair account of their reunion in the lobby of a Freiburg hotel, although anyone who wants to catch the emotional momentum of that occasion would need to read the original letters.

Arendt's great books such as The Origins of Totalitarianism and The Human Condition were written after that reunion. They pursued themes that came out of the upheaval she lived through, and all in some way reflected the question, how could a genius like Heidegger also be a potential murderer? She set a positive agenda. We should think for ourselves - risk being pariahs - if we want to resist mass brainwashing. We need labour (to survive), work (self-cultivation) and action (disinterested political and social engagement) in equal measure to pursue the good life. We can't just live, Heideggerishly, in our heads, for ourselves. And we must learn to forgive.

Whipping up hatred against her in the 1960s, Arendt's critics decided her early love for Heidegger had alienated her from her race and blinded her to the truth of a victimised people. What Maier-Katkin shows in this twin biography is that, on the contrary, her lifelong bond with that problematic German helped her define love as the supreme human - and political - value. One can never forgive the deed, she wrote, but one can, out of love, forgive the person. Otherwise how can a good world ever move on?

Lesley Chamberlain, author of 'Nietzsche in Turin', is working on a study of Van Gogh and Heidegger