Antonia Lolordo presents an original interpretation of John Locke's conception of moral agency--one that has implications both for his metaphysics and for the foundations of his political theory. Locke denies that species boundaries exist independently of human convention, holds that the human mind may be either an immaterial substance or a material one to which God has superadded the power of thought, and insists that animals possess the ability to perceive, will, and even reason--indeed, in some cases to reason better than humans. Thus, he eliminates any sharp distinction between humans and the rest of the animal kingdom. However, in his ethical and political work Locke assumes that there is a sharp distinction between moral agents and other beings. He thus needs to be able to delineate the set of moral agents precisely, without relying on the sort of metaphysical and physical facts his predecessors appealed to. Lolordo argues that for Locke, to be a moral agent is simply to be free, rational, and a person. Interpreting the Lockean metaphysics of moral agency in this way helps us to understand both Locke's over-arching philosophical project and the details of his accounts of liberty, personhood, and rationality.