Because a small error in the beginning grows enormous at the end, as the Philosopher remarks in Book 1 of On the Heavens and the World, and being and essence are the first things to be conceived by our understanding, as Avicenna declares in Book 1 of his Metaphysics, in order to avoid falling into error about them, and to reveal their difficulties, we should see what are signified by the names of being and essence, how these are found in various things, and how they are related to the logical intentions of genus, species, and difference. And since we need to arrive at the cognition of simple components from the cognition of what they compose, and from those that are posterior to those that are prior, so that the discussion may suitably progress from the easier subjects, we should proceed from the signification of the name of being to the signification of the name of essence.