Sex differentiation is a hot button topic in current public discussion, with some saying that the differences between male and female are of little consequence to the concept of gender in society. In this article, Cook explores the question of whether sex is substance or accident in accordance with Aristotelian philosophy as a way to gain a clearer understanding of sex differentiation that modern times sorely need. Drawing on two thousand years of intellectual tradition through both Aristotle and St. Thomas Aquinas, Cook provides a concise understanding of the ‘why’ of classic understandings of sex differentiation and makes a strong defense of the position that Aristotelian philosophy favors a concept of sex as immutable.