This article explores the belonging of books M and N to the general program of Aristotle's Metaphysics. Books XIII (My) and XIV (Ny) have remained in the background of Metaphysics, as a kind of editorial aggregate, which can be dispensed with for the understanding of the Aristotelian proposal. In this article, however, a different starting point is assumed, which consists in integrating these books into the core of the proposal of Metaphysics, understanding that they are key to completing the panorama of the general theory of substance, or ousiology, which guides all metaphysical books. Thus, the discussion of numbers and principles within M and N concludes the question of substance as cause and principle, announced from the beginning of Metaphysics and developed in its central books.