This book has developed gradually over a period of years, culminating in a series of academic appointments. In 1938 I treated part of the subject in a course of lectures on "Language and Fact" at the University of Oxford. These lectures formed the basis for seminar courses at the University of Chicago in 1938-9 and the University of California at Los Angeles in 1939-40. The discussions at the two seminars did much to widen my conception of the problems involved and to diminish the emphasis which I originally placed on the linguistic aspects of the subject. I have to express a collective obligation to those, both Professors and pupils, who, by detailed friendly criticism, helped (I hope) in the avoidance of errors and fallacies. More especially at Chicago, where the seminar was often attended by Professors Camap and Morris, and where some of the graduate students showed great philosophic ability, the discussions were models of fruitful argumentative cooperation. Mr. Norman Dalkey, who attended both seminars, has since read the whole book in manuscript, and I am greatly indebted to him for his careful and stimulating criticism. Finally, during the summer of 1940, I prepared these William James Lectures partly from accumulated material, and partly from a re-consideration of the whole subject.