"I am not much given to emotions", wrote the 30-years old mathematician Hans Hahn in 1909 to the physicist Paul Ehrenfest, who lived in St. Petersburg, "but to a friend who is as far away as you are, I confess it: sometimes, when I have attempted to dip into the metaphysics of Aristotle, I have felt awe-struck, and I much regret to lack the opportunities to ponder these things in depth, as I have pondered the calculus of variations."¹ In due course, however, Hahn managed to create those opportunities for philosophical studies for which he longed so much; and when he died some twenty-five years later, the physicist Philipp Frank could describe him as the true founder of the Vienna Circlet.²