Aristotle is sometimes described as the father of natural law, but how well does this description hold up to scrutiny? In this thesis, I attempt to offer a historically accurate (and philosophically viable) reconstruction of Aristotle’s ethical and political thought, with special reference to the role of natural law, and consequently to determine his proper place in the physis–nomos debate between ‘naturalists’ and ‘conventionalists’ in ancient Athens. This debate is seen as a sort of spiritual predecessor to the modern debate between natural law theorists and legal positivists in the philosophy of law, and the problems discussed in this thesis can therefore be considered as having perennial relevance. I aim to uphold the ‘traditional’ reading of Aristotle as a classical natural law theorist against thinkers like Hans Kelsen and Bernard Yack, who seem to interpret him as something like a conventionalist and a forerunner of legal positivism. In this endeavour, I am on the side of Fred Miller, Jr. and greatly helped by the commentaries of Thomas Aquinas. This thesis is also in part a response to the work of Tony Burns, who has recently argued for a ‘synthesis’ between the readings of Kelsen and Miller. On his interpretation, Aristotle holds to a ‘formal’ conception of natural law, which is similar to legal positivism in so far as it involves the view that the positive law of the state can never come into conflict with natural law. This is because Aristotle, according to Burns, does not conceive of natural law as a standard of justice by which the laws and customs of a society can be critically evaluated. Through a close reading of such works by Aristotle as the Nicomachean Ethics, the Politics, and the Rhetoric, I attempt to renew and strengthen the view that Aristotle, on the contrary, does conceive of natural law as the standard of justice. Along the way, I offer arguments on such topics as the relationship between Aristotle’s practical philosophy and philosophy of nature, the place of moral absolutes in what is known as Aristotelian virtue ethics, and the very foundation of what makes certain actions right or wrong (just or unjust) on Aristotle’s view.