•Why it earns a slot
On Liberty gave the English-speaking world its most precise and influential formulation of the harm principle, a standard that continues to anchor debates on free speech, paternalism, and the limits of law more than 160 years after its publication.
On Liberty argues that the only legitimate reason for society or government to restrict an individual's freedom is to prevent harm to others; self-protection is the sole valid justification for coercion. Mill defends freedom of thought and expression absolutely, contending that silencing any opinion robs humanity of truth or of the vital contest that keeps truth alive. He then extends this principle to individuality in action, warning that the growing tyranny of social conformity threatens human development as surely as political despotism.
This distillation is written from the freely available original, which is always the better read when you have the time: gutenberg.org.