Nineteenth-century thought, as many commentators have remarked, was profoundly influenced by notions of progress, by a consciousness of history, by the idea of evolution. This new understanding had a great impact on the theory of mind developed by two prominent English philosopher-scientists, Herbert Spencer and his disciple, John Hughlings Jackson. In this paper I will examine what seems to me to be the unjustly neglected work of Herbert Spencer. In a second paper, to appear in the next issue of this journal, I will examine the philosophical neurology of John Hughlings Jackson. When, in 1650, Descartes died of a chill brought on by the necessity of rising early in the bitter Stockholm winter he left among his papers the unpublished manuscript of a treatise on man.' It is well known that he refrained frompublishing the Treatise of Man during his lifetime because of the odium theologicum into which Galileo had fallen; for it is not difficult to give the Treatise an atheistic interpretation. At the beginning of the book, referring to the paradigmatic humans who form its subject matter, Descartes wrote: "I assume their body to be but an earthen machine ... we see clocks, fountains, mills and similar machines." In the case of "real" humans, of course, Descartes indicated that attached to the machine was an immortal soul. How far Descartes masked his real opinion has long been a matter of controversy.2 In the case of infra-human animals, however, Descartes is clear. The Discourse on Method, part V, says that they are to be regarded as soulless machines. This crucial distinction between animals and men was destined to become exposed, like a geological fault, when theories of evolution began to be developed in the late eighteenth and early nineteenth centuries.