On Wednesday 18 October 1911, Bertrand Russell was having tea with CK Ogden in his rooms at Trinity College Cambridge when they were interrupted by a complete stranger. As Russell wrote to Lady Ottoline Morrell, "an unknown German appeared, speaking very little English but refusing to speak German. He turned out to be a man who had learned engineering at Charlottenberg, but during his course had acquired by himself, a passion for the philosophy of mathematics and has now come to Cambridge on purpose to hear me."
The German turned out to be an Austrian called Ludwig Wittgenstein. Things would never be quite the same again for either of them - just three months later Russell wrote to Lady Morrell: "I love him & feel he will solve the problems I am too old to solve ... He is the young man one hopes for."
Dr John Preston from the University of Reading has set up a website to chronicle Wittgenstein's philosophical development on a day-by-day basis. As he says, "from 18th October 2011, the site will slowly build up information covering events up to the end of 1926, at the same pace as these events took place, exactly one century ago". Should be well worth a look.