  Spinoza’s insistence that human beings not be treated as a dominion within a dominion includes a commitment to ethical naturalism also. Just as he insists that the human mind must be explicable in terms of the laws which govern nature, so he insists that ethical properties, which he sometimes characterizes as human “modes of thinking,” be explicable in terms of natural ones. The theory of the affects serves Spinoza’s ethical naturalism by introducing explanations of ethical concepts, most importantly the concepts of good, evil, and perfection, in psychological terms. In his ethics, Spinoza in some way “retains these words,” although he may be understood to do so under some formal refinement or revision of them (See IV Preface). So his discussions of good and evil and of human perfection in Part III provide the basis for the formal ethical argument which follows in Parts IV and V. 