Canonic forms the introduction to the system and is contained in a single work entitled The physical part includes the entire theory of Nature: it is contained in the thirty-seven books and, in a summary form, in the letters. The ethical part deals with the facts of choice and aversion: this may be found in the books , in the letters, and in his treatise The usual arrangement, however, is to conjoin canonic with physics, and the former they call the science which deals with the standard and the first principle, or the elementary part of philosophy, while physics proper, they say, deals with becoming and perishing and with nature; ethics, on the other hand, deals with things to be sought and avoided, with human life and with the end-in-chief.