The 17th Century English philosopher Thomas Hobbes is now widely regarded as one of a handful of truly great political philosophers, whose masterwork Leviathan rivals in significance the political writings of Plato, Aristotle, Locke, Rousseau, Kant, and Rawls. Hobbes is famous for his early and elaborate development of what has come to be known as “social contract theory”, the method of justifying political principles or arrangements by appeal to the agreement that would be made among suitably situated rational, free, and equal persons. He is infamous for having used the social contract method to arrive at the astonishing conclusion that we ought to submit to the authority of an absolute—undivided and unlimited—sovereign power. While his methodological innovation had a profound constructive impact on subsequent work in political philosophy, his substantive conclusions have served mostly as a foil for the development of more palatable philosophical positions. Hobbes's moral philosophy has been less influential than his political philosophy, in part because that theory is too ambiguous to have garnered any general consensus as to its content. Most scholars have taken Hobbes to have affirmed some sort of personal relativism or subjectivism; but views that Hobbes espoused divine command theory, virtue ethics, rule egoism, or a form of projectivism also find support in Hobbes's texts and among scholars. Because Hobbes held that “the true doctrine of the Lawes of Nature is the true Morall philosophie”, differences in interpretation of Hobbes's moral philosophy can be traced to differing understandings of the status and operation of Hobbes's “laws of nature”, which laws will be discussed below. The formerly dominant view that Hobbes espoused psychological egoism as the foundation of his moral theory is currently widely rejected, and there has been to date no fully systematic study of Hobbes's moral psychology.