I am deeply indebted to the thoughtful interest of President William Oxley Thompson in suggesting, and to the trustees of the Ohio State University in sanctioning, my relief from routine duties in order to bring this work to completion. I am indebted to my colleagues in the Department of Philosophy, Doctors A. E. Avey, A. R. Chandler, and R. D. Williams, for their never failing interest, and, especially, for the cheerful alacrity with which they have relieved me of my teaching duties in order that I might finish and publish this book. For a number of stylistic suggestions I am indebted to Doctor Chandler. Doctors J. E. Creighton, Chandler and Avey have assisted me materially in proofreading.