Liberty is the condition of being able to act without illegitimate interference. The disputes concern what counts as interference and what makes it illegitimate.

Negative and positive

Isaiah Berlin drew the standard distinction in 1958. Negative liberty is the absence of obstacles imposed by others — you are free to the extent that nobody stops you. Positive liberty is the capacity to act on your own reasons and be the author of your life.

Berlin warned that positive liberty has a dangerous structure. Once freedom means acting from your real interests, someone can claim to know those interests better than you do, and coercion can be presented as liberation. Defenders reply that negative liberty alone is thin: a person with no resources, education or options is unobstructed and not meaningfully free.

Mill's line

John Stuart Mill gave the most influential statement in On Liberty (1859). The only purpose for which power can rightfully be exercised over a member of a civilized community, against their will, is to prevent harm to others. A person's own good is not a sufficient warrant.

Mill defended free expression on separate grounds. A suppressed opinion may be true; if false, it sharpens the truth by opposition; and a belief never contested becomes dead dogma held without understanding of why it is held.

Republican freedom

A third tradition, revived by Philip Pettit, defines freedom as the absence of domination rather than of interference. A person living at the mercy of a benevolent master is unfree even while nothing is being done to them, because the capacity for arbitrary interference is itself the loss. This captures intuitions about dependence that the negative account misses.

Liberty connects to justice, to the social contract that defines the terms of legitimate authority, and to free will, which asks about a different kind of freedom altogether.