This chapter principally considers the scheme of the cardinal virtues in ST 2-2, which Aquinas developed in order to organize comprehensively the subject matter of ethics. It discusses key differences in ethical method between Aristotle and Aquinas Aquinas develops Aristotle's ethical theory in the EN by resolving difficulties inherent in the EN, drawing on principles taken from Aristotle to do so. He does so as part of a project that he regards as primarily speculative, accounting for the truth of things, and not merely practical, aiming at the good. Ethical theory, if it is true, must have a formal structure, consistent with the best contemporary accounts of the world, and that admits of being more deeply articulated as investigation proceeds and deepens. Aquinas's virtue ethics has a clever, deep, and compelling rational structure. Its claim to truth depends crucially on the claims to truth of Aristotelian natural philosophy and metaphysics.