Who was the schoolboy who triggered Adolf Hitler's hatred of the Jews? Kimberley Cornish points the finger at one of the great thinkers of the 20th century.

Soon after it enters Austria on its journey from Germany to the Black Sea, the River Danube flows into a wide valley and passes through the city of Linz. Trade routes have criss-crossed at Linz since Roman times and its ancient buildings chronicle many centuries of Austrian power. Nowadays Linz is a provincial capital and industrial centre, but it holds a particular niche in the violent history of the 20th century.

Adolf Hitler went to school there and later revealed in his racist testament, Mein Kampf, that his initial feelings of anti-semitism had been stirred by an unnamed Jewish schoolboy in the city. This was the first link in the chain of hatred that led to Auschwitz.

One of the mysteries of the 20th century has been the identity of this boy who turned Hitler into the killer of 6m Jews. Hitler hated him with an all-consuming fire and he was perhaps the very first victim of the future Fuhrer's bullying. But who was he? Did he survive the Holocaust?

Although there is not much to go on in Hitler's book, it is possible to make a fair bet who he was, and this "fair bet" is astonishing. The Jew of Linz grew into a seminal 20th-century thinker, a logician and philosopher of the first rank, a man as extraordinary as Hitler himself. His name was Ludwig Wittgenstein.

There could be even more to this story. Nazism and the Holocaust were both influenced by Wittgenstein - but did he also assist the Stalinist triumph in eastern Europe, because he was so desperate to defeat Hitler that he secretly resorted to evil means that seriously damaged Great Britain, his adopted country?

Piecing the strands of the story together involves pioneer detective work. The implications may, on the face of it, seem utterly implausible. To show its plausibility, we must first look at the era that spawned Hitler and his fellow schoolboy and find out how they met each other.

ADOLF HITLER was born on April 20, 1889, in the frontier town of Braunau on the River Inn, which separated the Austro-Hungarian empire from Bavaria. Six days later, a rich socialite called Leopoldine Wittgenstein gave birth to Ludwig, her eighth child, in the glittering imperial capital, Vienna.

The two boys were born in the dying decades of a glorious empire. Once Austria had guided Europe's destiny. Now Prussia had usurped it as the dominant Germanic power; Austria's emperor, Franz Josef, ruled over a vast rump of nationalities and territories that sprawled from Bosnia to Ukraine. This empire was huge, weak and unwieldy, and its aristocratic elite had no great purpose beyond self-preservation and the enjoyment of Vienna's prosperity and gentle way of life.

"There was no ambition for world markets or world power," Robert Musil, the Viennese novelist, observed. "Ruinous sums of money were spent...