This book would have been very much more obscure and more frequently mistaken than it is without the generous and patient help of four of my friends. My most grateful thanks are due to Miss Joy Coles and Miss Patricia Gleeson for their many warnings of passages which they, as non-philosophers, found hard or impossible to understand, and for their help in making these passages clearer; also to Professor D. J. O'Connor and Mr. R. F. Atkinson for their thorough-going criticisms and comments from a professional point of view. I am equally grateful to another friend, Mrs. Margaret Harvey, for having undertaken the tedious and difficult task of compiling the index.
I need hardly add that any obscurities and mistakes that remain are entirely my own responsibility.
Keele, May, 1958.
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