First published in 1962 tries to explain scientific thought from its historical and psychological origins. The depth of psychology of today rather than traditional epistemology is needed in order to understand the problems of knowledge. Reality is the first problem of the scientist; it is exemplified by the idea of object, or of matter. The development of this idea from its Greek beginnings is traced and the unconscious mechanisms that underlie our thought processes -of abstraction, generalisation, etc.- are made manifest. The second problem is that of truth; it is illustrated by examples from the history of mathematics and of logic. Again, the ‘psychology’ of what we accept as truth is made explicit. Scientific method is the intellectual safeguard for the criteria of truth and reality. Instead of traditional induction, the creative view of scientific activity must be accepted. This book is a must read for scholars and researchers of philosophy of science and philosophy in general.