The Society for the Advancement of the Scientific World Conception has done me a great honor by inviting me to be the Sixth Vienna Circle Lecturer. The invitation has also stirred some deep emotions. A central figure of the Vienna Circle, Rudolf Carnap, was my revered teacher of philosophy at the University of Chicago in 1948–9 and later an informal adviser when I wrote a doctoral thesis at Yale University on inductive logic, and he was a friend during those years and thereafter. I was not a disciple, but Carnap did not demand discipleship as a condition for admission to his seminars or to his friendship. He seemed to be baffled by the fact that despite my interest in mathematical logic and theoretical physics I proclaimed myself a metaphysicician and had even published an article in the first issue of The Review of Metaphysics. Carnap (1937, pp. 51–52) formulated a "principle of tolerance" as a philosophical maxim concerning rules of language ("in logic, there are no morals"), but he practiced a human and highly moral version of the principle of tolerance in his profoundly liberal social commitments and in his relations with his students. If he were here tonight, I would wish for his tolerance of the lapses of rigor and the flights of speculation to which he would be exposed.