  Because of the absence of a priori demonstrative premises, a moral philosopher arguing from matters of fact had to have detailed knowledge of the principles of action in human nature and of moral psychology as well as arguments to show how these principles had bearing on virtue and vice. Human beings, according to Butler, had within their nature various instincts and principles of action: desires for particular pleasures, benevolence, self-love, and conscience. The practitioner of moral science aimed to discover what these principles are and the ways in which these desires, reasons, and motivations fit together. This is difficult in practice since the actions that we observe in ourselves and in others almost always draw on more than one principle. For example both self-love and the particular passions often go into the motivation or justification for particular actions. Consequently the moral scientist could be said to bring a version of Locke’s negative program of underbrush clearing in the theory of knowledge to moral philosophy, by analyzing and disambiguating principles, motivations, and passions which give rise to actions which we judge to be virtuous and vicious. At the same time Butler’s way of thinking about morality as dictated by nature had strong resonances with the Stoics and other ancient moralists.