One of the sciences comprised in Hume’s Science of Human Nature is metaphysics, understood as the science of the most general features of the world as it appears to us. I show how Hume’s metaphysics in the Treatise can be “methodized, or reduced to general principles.” Those principles are: (I) The Contradiction Principle: The distinctly conceivable implies no contradiction. (II) The Possibility Principle: What implies no contradiction is possible. (III) The Conceptual Separability Principle: Things are different if and only if separable in conception. On these principles the rest of Hume’s metaphysics is based, including his atomistic ontology and his denial of necessary connection: only single, individual, simple, unaltering, determinate, particular, metaphysically independent things exist.