Prefaces are very difficult to write and, like many authors, I tend to leave their composition until the last possible moment. I always hope, in vain, that inspiration will strike and that I shall be able to give some novel twist to what is customarily a dry recital of all the people who have in any way contributed to a work but who are in no way responsible for its shortcomings. Of course, I should greatly prefer to be able to blame my shortcomings upon others but, although this book has been decades in the making, on the whole I have worked by myself, in my own way, and at my own tempo. There are thus only a few individuals I should like to mention: but they have, in various ways, been important to me. One particular debt is to the late Nicolai Rubinstein who, over many years, never failed to stir my conscience at the tardy progress of this book and never failed to encourage me to bestir myself. I bitterly regret that he did not live to see the finished work, but hope that his many surviving friends and pupils will not think it unworthy of his memory.