This chapter analyzes Aquinas’s account of the principles of metaphysics – both the causal principles of common being and the scientific principles (i.e., the per se nota propositions) from which metaphysical reasoning proceeds. Because the subject-matter of Aquinas’s metaphysics has real efficient, formal, and final causes, it is possible to formulate corresponding real causal definitions of that subject, which can, in turn, serve as terms used to construct the primitive principles of metaphysical science. This means is that it is possible for Aquinas’s metaphysics to be “scientific” in the strongest sense in which he understood the word – that is, that Aquinas’s metaphysics can meet even the most rigorous standards of Aristotelian episteme.