Many friends have helped me with this book. They have directed my reading, corrected my misconceptions, primed me with facts, suggested turns of phrase. Often they have done so unawares, ignorant of the investigations I was pursuing. How can I acknowledge my debt to each of them? Who can keep account of all the written and spoken words from which his own thought has been distilled? So I can name in gratitude only those who have read all or parts of the manuscript and helped me to eliminate many defects literary, scientific and philosophical. These are my wife, Dr. Dorothy Kapp; Dr H. Gruneberg of the Department of Zoology and Comparitive Anatomy at University College, London; Dr Wilkinson of the Department of Biology at University College, Swansea; Mr. W. B. Gallie of the Department of Philosophy at University College, Swansea; and Professor M. T. Smiley fo the Department of Greek, University College, London