Herbert Spencer’s Principles of Psychology, published four years before the appearance of The Origin of Species by Charles Darwin, was the first attempt to elaborate a psychology from an evolutionary point of view. The author’s aim is to reconstruct the path that led Spencer to formulate the main hypothesis of his pioneering work. This paper focuses chiefly on the debate which arose in Great Britain during the first half of the 1850s concerning the so called "development hypothesis". From the discussions he had with G.H. Lewes and T.H. Huxley on the theories of Lamarck and on the proposals made by a book that was extraordinarily popular at the time, Spencer drew conclusions which he considered well-grounded even after the publication of Darwin’s book.
Keywords: Associationism, Creationism, Phrenology, Lamarckism, Mind, Psychology