Originally published in 1965, this title is a series of exploratory essays on approaches to thinking. The central topic is the relation of processes of an associative kind (sometimes irrational, in so far as they are not enmeshed with a world of shared experience) to those involving some degree of reference to a common world and hence forming the basis of constructive, critical and logical thought. This theme ran through a good deal of psychological controversy at the time. It is a very old theme that had been dealt with many times and in many ways in the course of its history. One might have chosen to discuss approaches to it other than those considered in the present volume. These, however, were selected for their bearing on one another, and because they formed an interesting part of the background to contemporary psychological theory of the time.