 Locke’s greatest philosophical work, An Essay Concerning Human Understanding, is generally seen as a defining work of seventeenth-century empiricist epistemology and metaphysics. The moral philosophy developed in this work is rarely taken up for critical analysis, considered by many scholars of Locke’s thought to be too obscure and confusing to be taken too seriously. The view is not only seen by many commentators as incomplete, but it carries a degree of rationalism that cannot be made consistent with our picture of Locke as the arch-empiricist of his period. While it is true that Locke’s discussion of morality in the Essay is not as well-developed as many of his other views, there is reason to think that morality was the driving concern of this great work. For Locke, morality is the one area apart from mathematics wherein human reasoning can attain a level of rational certitude. For Locke, human reason may be weak with regards to our understanding of the natural world and the workings of the human mind, but it is exactly suited for the job of figuring out human moral duty. By looking at Locke’s moral philosophy, as it is developed in the Essay and some of his earlier writings, we gain a heightened appreciation for Locke’s motivations in the Essay, as well as a more nuanced understanding of the degree of Locke’s empiricism. Further than this, Locke’s moral philosophy offers us an important exemplar of seventeenth-century natural law theory, probably the predominant moral view of the period.