 Likely no classic philosophical work is cited more often in the public health ethics literature than John Stuart Mill’s influential essay, “On Liberty” (Mill 1869). In that essay, Mill articulates and defends what has come to be called ‘the harm principle,’ although Mill himself never used the term. The harm principle has been interpreted as holding that the only justification for interfering with the liberty of an individual, against her will, is to prevent direct, non-consensual harm to others. The harm principle is used to both challenge and support public health policies.