In the academic year 1953/54, I held lectures and seminars on contemporary analytical philosophy in the USA and England as a visiting professor at the University of Vienna. This opportunity was made possible by an invitation from the philosophical faculty, primarily due to the initiative of retired associate professor Dr. Victor Kraft, and I was granted a Fulbright scholarship for this purpose by the American State Department. The present book is a result of this activity. However, it would hardly have been completed in such a short time without the expert and enthusiastic collaboration of Dr. Paul Feyerabend, to whom I would like to express my heartfelt thanks. Fragments of a lecture manuscript were only available for the first three chapters. Dr. Feyerabend produced the missing sections ex post facto based on the notes he took as an exceptionally active listener during the lectures.