The book Lambda —which is critically edited, explained, and translated here— represents the culmination of the treatise Metaphysics, as in it Aristotle provides us with knowledge regarding the divine first principle, upon which, as he himself states, heaven and nature depend. After a long investigation into how we can be led to the discovery of a self-existent, eternal, and unchanging substance, Aristotle teaches us in the book Lambda what and how many these divine substances are. He leads us from the celestial bodies, which are responsible for the orderly and unending generation of perishable substances, to their souls, which are responsible for their eternal movements, and from the souls of the celestial substances, he leads us to the motionless eternal substances, namely the self-existent intellects, the first of which is the Good itself, the ultimate final cause and the beginning of the causal cosmic chain.