The present volume is the product of several years of collaboration at a distance between two people who both knew Yres R. Simon personally and admired his work. The question raised by Simon more than half a century ago, when this book was first published, are still with us: What is the nature of knowledge? What kind of activity is it to know? What is involved in the development of human knowledge? If one had to describe Simon's accomplishment by reducing it to a single point, what he succeeded in showing was that an ontology of knowledge based on common experience disproves all idealism and leads to realism by strictest necessity.