Science is about real things. That is what makes it knowledge rather than fantasy. But what things are real? What are the fundamental items with which science is concerned? That is the question of ontology, to which Aristotle devoted much attention. One of his ontological essays, the Categories, is relatively clear; but most of his ontological thought is to be found in the more obscure Metaphysics. ‘Reality’ attempts to determine what Aristotelian substances are. The first and plainest examples of substances are animals and plants and perhaps also artefacts. In general, perceptible things — middle-sized material objects — are Aristotle’s basic realities and the things with which science is principally concerned.