The Cambridge History of English and American Literature in 18 Volumes (1907–21). Volume XIV. The Victorian Age, Part Two. I. Philosophers. § 10. On Liberty. The essay On Liberty—the most popular of all his works—is an eloquent defence of the thesis “that the sole end for which mankind are warranted, individually or collectively, in interfering with the liberty of action of any of their number, is self-protection,” but, as an argument, it meets everywhere with the difficulty of determining the precise point at which the distinction between self-regarding and social (even directly social) activity is to be drawn. Sir James Fitzjames Stephen, accepting Mill’s utilitarian criterion, raked his positions with a fire of brilliant and incisive, if unsympathetic, criticism in Liberty, Equality, Fraternity (1873).   34