In this essay, V. Bradley Lewis gives an introduction to Plato’s philosophy, and its contribution to natural law theory. Although the term “natural law” occurs only once in Plato’s writings, many of his dialogues were designed to call into question the prevailing understanding of nature and law as oppositional. What emerges from Plato is the idea of nature as normative for human affairs as the rule of reason. In his dialogues, Plato suggests that the political art, which is intrinsically legislative, has as its aim the good of the soul of the citizen under its authority. He also suggests that this law can be rationally discerned, while conceding that lawgivers can be mistaken. For Plato, the laws of the city should be guided by a rational and natural ordering of divine and human goods, but discovering the ordering of these goods – and the laws required to promote them – itself depends on reason trained in virtue. In his consideration of the challenges and promises of reason (or intelligence), Plato warns against the dangers that atheism poses to the city, and provides a lengthy natural theology in response. The response serves, on the one hand, to explain the existence of divine souls, but ultimately also the order of goods in the human soul that is of the greatest importance to Plato’s understanding of natural right. By demanding that human laws conform to a natural and rationally discernable standard of justice, Plato helped to initiate a natural law tradition that takes law to be more than a merely arbitrary cultural institution.