Although this book was first published more than half a century ago, the questions raised by Simon - what is the nature of knowledge? what kind of activity is it - to know? and what is involved in the development of human knowledge - are still with us. Amidst the confusion of current debates, Simon's treatment of these questions is clear indeed. To answer the first question, he develops the theory of "intentional" existence and its conditions. To solve the second problem, he explains at length the nature of immanent action and the sense in which knowledge is an activity and a quality. And with regard to the third question, he does not hesitate to face its many difficulties, including the necessity and the role of experimental knowledge, the role of material things in producing sensation, the contribution of sensible knowledge in the formation of concepts, the significance of judgements in the knowledge of truth, and the various forms of the active development of the mind.
But if one had to describe Simon's objectives by reducing them to a single point, what Simon aimed to show was that an ontology of knowledge based on common experience disproves all idealism and leads to realism by strictest necessity.