The Logic of the Moral Sciences

Mill insists that the uniformity of nature extends to human beings. The same causes, operating on the same characters, will always produce the same effects. This is not determinism in the sense that eliminates freedom: Mill distinguishes being caused from being compelled, arguing that a person acts freely when the cause of their action is their own character and desires, even if those character and desires were themselves shaped by prior causes. Freedom and causal law are compatible; free will as a source of uncaused action is neither possible nor desirable.

Between general psychology (the laws of mind) and sociology (the laws of society) Mill proposes ethology — a middle-level science of character formation, investigating how circumstances produce different kinds of character in different individuals, classes, and nations. Though Mill's own ethological programme was never completed, it anticipates the social-scientific study of socialization, culture, and the formation of personality that would become central to twentieth-century sociology and anthropology.

For complex social phenomena, direct experimentation is impossible — we cannot run controlled experiments on whole societies. Mill's solution is the 'inverse deductive method' (what later became known as the hypothetico-deductive method): propose a theoretical account derived from psychological laws, deduce what observable patterns it would predict, and test those predictions against historical and statistical evidence. This remains the dominant methodology of the social sciences, adapted and refined through a century and a half of practice.

Book VI of A System of Logic, "On the Logic of the Moral Sciences," is the founding document of social-scientific methodology. It directly influenced Auguste Comte, Herbert Spencer, and the subsequent development of sociology and economics.