My thesis intends to show how a detailed study of the issue of sexual equality in the correspondence between Auguste Comte and john Stuart Mill can shed light (1) on the general epistemological, methodological, political, social, and moral disagreements existing between Comte and Mill, and (2) on the evolution of Mill's arguments for the emancipation of women. I start with a summary of the circumstances which led Comte and Mill to address the topic of sexual equality and I introduce their respective views on the subject prior to the start of the correspondence in the early 1840s. I then review the various biological arguments adduced by Comte in support of his belief in the natural intellectual inferiority of women and single out his commitment to phrenology as a crucial element for his case for women's subjection. I present Mill's rejoinder to Comte's phrenological case and explain how it relates to Mill's defense of associationist psychology and the conception of the "logic of the moral sciences" developed in his System of logic. I then turn to Comte's sociological arguments for the subjection of women. I show how they in fact rely on a biologically inspired conception of human "development", and present the interpretation of the historical record Mill opposes to Comte. The subsequent chapter introduces Mill's pet project of Ethology, which he thought would provide proper knowledge of human nature capable of adjudicating the sexual equality debate. Furthermore it analyses the methodological obstacles which prevented Mill from developing this new 'science of the formation of human character'. I conclude by showing how his failed attempt at founding Ethology forced Mill to find (most notably in his Subjection of Women) alternative arguments for the emancipation of women and investigate how they tally with his mature ethical and social views on human nature.