 He offers his explanation in last chapter of his Metaphysical Disputations, which is dedicated to beings of reason (entia rationis). This disputation is independently interesting because it engages a question which has continued to fascinate metaphysicians down to the present day, though it has taken various forms in various periods of inquiry. This is the question of how we are to speak (or think) about things that are not. If something is unavailable to be spoken or thought about, then, presumably, we do not speak or think about it. After all, as Suárez notes, non-existing beings of reason about which we pretend to think or speak