Medieval Sourcebook: Aquinas: Ente et Essentia On Being and Essence (DE ENTE Et ESSENTIA) by Thomas AquinasPrologue A small error at the outset can lead to great errors in the final conclusions, as the Philosopher says in I De Caelo et Mundo cap. 5 (271b8-13), and thus, since being and essence are the things first conceived of by the intellect, as Avicenna says in Metaphysicae I, cap. 6, in order to avoid errors arising from ignorance about these two things, we should resolve the difficulties surrounding them by explaining what the terms being and essence each signify and by showing how each may be found in various things and how each is related to the logical intentions of genus, species, and difference.
Since we ought to acquire knowledge of simple things from composite ones and come to know the prior from the posterior, in instructing beginners we should begin with what is easier, and so we shall begin with the signification of being and proceed from there to the signification of essence.
