The three basic principles of Plotinus' metaphysics are called by him "the One" (or, equivalently, "the Good"), Intellect, and Soul (see V 1; V 9.). These principles are both ultimate ontological realities and explanatory principles. Plotinus believed that they were recognized by Plato as such, as well as by the entire subsequent Platonic tradition. The One is the absolutely simple first principle of all. It is both "self-caused" and the cause of being for everything else in the universe. There are, according to Plotinus, various ways of showing the necessity of positing such a principle.