The book’s final essay, “John Stuart Mill and the Ends of Life,” looks at an individual who has drawn such lines. Mill, whose father’s teaching enabled him to read Greek at age five and to know algebra and Latin by the age of nine, established modern liberalism with the publication of On Liberty in 1859. Berlin’s study of Mill reveals the extent and expanse of the British philosopher’s contributions to the theory of liberty.

As a thinker, writer, and political leader, Mill was concerned above all with the extension of individual freedom, especially freedom of belief and speech. Time after time, he argued for the right of unpopular, even dangerous, groups and individuals to express their views freely and openly. Mill passionately believed that human society needed variety and that tolerance of the ideas of others was necessary for this variety.

Mill believed that human beings wish to curtail the liberties of others for one of three reasons: the desire for power, a wish for conformity, and the belief that there is a single, universal answer to each important question or issue. The first two reasons are irrational, Mill argues, and the third has been proven wrong by history. History has repeatedly demonstrated that human knowledge is never complete and that different individuals, nations, or civilizations can have different goals, equally valid but not necessarily in harmony with one another.

Mill’s most important and enduring contribution, Berlin believed, was his constant, untiring insistence that it is the freedom to choose and to experiment that distinguishes human beings from the rest of nature. This, above all else, is what liberty is expected to preserve.