The terms and principles of Art have, as a rule, been understood by the artist in the form of technical processes and visual images, not in words. It is in words that they will become generally understood. It is in words that I propose to explain them in this book. I want to bring to definition what, until now, has not been clearly defined or exactly measured. In a sense this book is a contribution to Science rather than to Art. It is a contribution to Science made by a painter, who has used his Art in order to understand his Art, not to produce Works of Art. In a passage of Plato (Philebus, ^f 55) Socrates says: "If arithmetic, mensuration, and weighing be taken out of any art, that which remains will not be much."