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John Stuart Mill, b. May 20, 1806, d. May 8, 1873, the most influential British social and political thinker of the mid-Victorian period, left a permanent imprint on philosophy through his restatements of the principles underlying philosophical empiricism and utilitarianism. As a defender of individual liberty against state interference, and as an early advocate of women's equality, Mill continues to be of major significance.

The son of philosopher James Mill, John Stuart was early introduced to the Philosophical Radicals, or Benthamites, who actively pursued various social and political reforms along the utilitarian lines laid down by Jeremy Bentham. James Mill personally undertook the education of his precocious son, beginning with Greek at age three, with the aim of preparing him intellectually for eventual leadership of the group. The account of this educational experience appears in the younger Mill's Autobiography (1873). In 1823 he followed his father into the employ of the British East India Company, which he served continuously until 1858.

In his early twenties Mill experienced a "mental crisis," in which he was overcome by intense depression and plagued by doubts concerning the causes to which he had previously been devoted, including the Benthamite philosophy of which he had become a leading spokesman. He believed that his education had been unduly narrow, and also feared that his ability to experience emotional excitement was inadequate. Although this period passed, it left a permanent imprint on Mill. Although he remained a Benthamite, he revised his earlier beliefs in important respects.

In 1830, Mill was introduced to Harriet Taylor, a woman who was married and the mother of several children. They developed a deep, unconventional, and probably platonic friendship that resulted in marriage 21 years later, following the death of her husband. Mill attributed to his wife, who died in 1858, a decisive influence on all his later work. After serving one term in Parliament (1865-68), Mill retired to France, where he spent six months of every year up until his death.

Mill's earliest important philosophical work, the System of Logic (1843), contains a valuable discussion of the epistemological principles underlying empiricism. Five years later came the Principles of Political Economy (1848). Mill is mainly remembered today, however, for his contributions to ethical and social theory, in which his writings continue to exert an important influence. Utilitarianism (1863) was his effort to state and defend the view that "the greatest happiness of the greatest number" should be the aim of personal and legislative conduct. However, he revised Bentham's version of utilitarianism by enriching the concept of pleasure. Although both Mill and Bentham believed that pleasure constituted happiness, Bentham had argued that all pleasures, physical or intellectual, were of equal value; Mill argued for the superiority of the "higher" pleasures of the mind. The earlier On Liberty (1859) was his statement of the principle that self-protection alone can justify either the state's tampering with the liberty of the individual or any personal interference with another's freedom. Mill particularly urged this point of view with respect to freedom of thought and discussion. The Subjection of Women (1869), a topic on which Mill and Harriet Taylor had collaborated as early as 1832, is a classic essay on all aspects of female emancipation.