In his System of Logic (1843), Mill added to formal logic a system of evidentiary proof to show how conclusions about matters of fact were justified. He also updated Francis Bacon’s (1561–1626) analysis of causation, and built on David Hume’s (1711–1776) theory that causes are not logically connected to their effects, and that causal relationships are no more than constant conjunctions of types of events.

In Principles of Political Economy (1848) Mill identified a gap between what was measured in economics and human values, such as the preservation of the environment and limited population. He argued that the ideal economy would be made up of worker-owned cooperatives.

Mill’s On Liberty (1859) was his most contested work because it was an attack on the leveling effects of social opinion. Mill thought that democratic societies imposed conventions on their members that did not allow for much individual experimentation in life styles. His more conservative contemporaries objected to the freedoms of opinion he championed, as well as his idea that if what others consider a vice does not harm them, they have no right to interfere with an individual who practices it. His Utilitarianism (1861) argued for the greatest good for the greatest number of people, in which the greatest good is defined as happiness.

His On the Subjection of Women (1869) has endured as a classic feminist work. His last major work, Three Essays on Religion (1874), was a rational perspective on religion, but was neither agnostic nor atheistic. Mill reasoned that there probably was a God, but that the amount of human suffering in the world made it unlikely that God was very benevolent toward human beings.