"The lectures contained in this volume were given for the University of Edinburgh on Lord Gifford's foundation in the years 1933 and 1934. I have delayed their publication in the hope that with process of time I might, by further reading and thought, be able to expand and modify them, so as to make them more worthy of presentation to the public in the form of a book. This hope has been so meagerly realized that it now seems best to let them go forth, with all their imperfections on their head, hardly at all altered from the form in which they were delivered." --From the preface
