The title essay of this book was presented as a pair of lectures of the same title at Columbia University, March 26 and 28, 1968. They constituted the first of the John Dewey Lectures, which are to be delivered biennially. Dewey was professor of philosophy at Columbia from 1905 until 1930.

A week after I gave the second lecture, the two lectures appeared by permission of Columbia University as an article in the Journal of Philosophy. I have since corrected and expanded one remark; the improvement comes in pages 56/and 61, where seven lines of old text have given way to a page and a half of new. Michael Jubien found the error. I had given a single lecture under the same title at Yale and the University of Chicago as early as May 1967, but it was no near approach to what is now before us. My thoughts on the matter matured appreciably in the intervening ten months.