Since a small error in the beginning is great at the end, according to the Philosopher in Book I of On the Heavens and the World, while being and essence are the things first conceived by the intellect, as Avicenna says at the beginning of his Metaphysics, therefore, lest we happen to err through ignorance of them, in order to disclose the difficulty concerning these matters it must be said what is signified by the names “essence” and “being,” and how they are found in different things, and how they are related to the logical intentions, namely genus, species, and difference.
But since we must acquire knowledge of simple things from composite things and proceed from posterior things to prior ones, so that, beginning from easier matters, the discipline may become more suitable, therefore we must proceed from the signification of being to the signification of essence.