Herbert Spencer (27 April 1820 – 8 December 1903) was an English polymath active as a philosopher, psychologist, biologist, sociologist, and anthropologist. Spencer originated the expression "survival of the fittest", which he coined in Principles of Biology (1864) after reading Charles Darwin's 1859 book On the Origin of Species. The term strongly suggests natural selection, yet Spencer saw evolution as extending into realms of sociology and ethics, so he also supported Lamarckism.

Spencer developed an all-embracing conception of evolution as the progressive development of the physical world, biological organisms, the human mind, and human culture and societies. As a polymath, he contributed to a wide range of subjects, including ethics, religion, anthropology, economics, political theory, philosophy, literature, astronomy, biology, sociology, and psychology. During his lifetime he achieved tremendous authority, mainly in English-speaking academia. Spencer was "the single most famous European intellectual in the closing decades of the nineteenth century" but his influence declined sharply after 1900: "Who now reads Spencer?" asked Talcott Parsons in 1937.

Spencer was born in Derby, Derbyshire, England, on 27 April 1820, the son of William George Spencer (generally called George). Spencer's father was a religious dissenter who drifted from Methodism to Quakerism, and who transmitted to his son an opposition to all forms of authority. He ran a school founded on the progressive teaching methods of Johann Heinrich Pestalozzi and also served as Secretary of the Derby Philosophical Society, a scientific society which had been founded in 1783 by Erasmus Darwin, the grandfather of Charles Darwin.

Spencer was educated in empirical science by his father, while the members of the Derby Philosophical Society introduced him to pre-Darwinian concepts of biological evolution, particularly those of Erasmus Darwin and Jean-Baptiste Lamarck. His uncle, the Reverend Thomas Spencer, vicar of Hinton Charterhouse near Bath, completed Spencer's limited formal education by teaching him some mathematics and physics, and enough Latin to enable him to translate some easy texts. Thomas Spencer also imprinted on his nephew his own firm free-trade and anti-statist political views. Otherwise, Spencer was an autodidact who acquired most of his knowledge from narrowly focused readings and conversations with his friends and acquaintances.

Both as an adolescent and as a young man, Spencer found it difficult to settle to any intellectual or professional discipline. He worked as a civil engineer during the railway boom of the late 1830s, while also devoting much of his time to writing for provincial journals that were nonconformist in their religion and radical in their politics.

Spencer published his first book, Social Statics (1851), whilst working as sub-editor on the free-trade journal The Economist from 1848 to 1853. He predicted that humanity would eventually become completely adapted to the requirements of living in society with the consequential withering away of the state. Its publisher, John Chapman, introduced Spencer to his salon which was attended by many of the leading radical and progressive thinkers of the capital, including John Stuart Mill, Harriet Martineau, George Henry Lewes and Mary Ann Evans (George Eliot), with whom he was briefly romantically linked. Spencer himself introduced the biologist Thomas Henry Huxley, who would later win fame as 'Darwin's Bulldog' and who remained Spencer's lifelong friend. However, it was the friendship of Evans and Lewes that acquainted him with John Stuart Mill's A System of Logic and with Auguste Comte's positivism and which set him on the road to his life's work. He strongly disagreed with Comte.

Spencer's second book, Principles of Psychology, published in 1855, explored a physiological basis for psychology, and was the fruit of his friendship with Evans and Lewes. The book was founded on his fundamental assumption that the human mind is subject to natural laws and that these can be discovered within the framework of general biology. This permitted the adoption of a developmental perspective not merely in terms of the individual (as in traditional psychology), but also of the species and the race. Through this paradigm, Spencer aimed to reconcile the associationist psychology of Mill's Logic, the notion that the human mind is constructed from atomic sensations held together by the laws of the association of ideas, with the apparently more 'scientific' theory of phrenology, which locates specific mental functions in specific parts of the brain.

Spencer argued that both these theories are partial accounts of the truth: repeated associations of ideas are embodied in the formation of specific strands of brain tissue, and these can be passed from one generation to the next by means of the Lamarckian mechanism of use-inheritance. The Psychology, he believed, would do for the human mind what Isaac Newton had done for matter. However, the book was not initially successful and the last of the 251 copies of its first edition were not sold until June 1861.