The ideas thus far developed move in a direction whose further pursuit inevitably issues in materialism. Diderot, the editor of the _Encyclopedia of the Sciences, Arts, and Trades_ (1751-72), which gathered all the currents of the Illumination into one great stream and carried them to the open sea of popular culture, reflects in his intellectual development the dialectical movement from deism through skepticism to atheism and materialism, and was a co-laborer in the work which brought the whole movement to a conclusion, Holbach's _System of Nature_. Two decades, however, before the latter work, the outcome of a long development of thought, appeared, the physician La Mettrie[1] (1709-51) had promulgated materialism, though rather in an anthropological form than as a world-system, and with cynical satisfaction in the violation of traditional beliefs--in his _Natural History of the Soul_, 1745, in a disguised form, and, undisguised, in his _Man a Machine_, 1748--and at the same time (_Anti-Seneca, or Discourse on Happiness_, 1748) had sketched out for Helvetius the outlines of the sensationalistic morality of interest. While ill with a violent fever he observed the influence of the heightened circulation of the blood on his mental tone, and inferred that thought is the result of the bodily organization. The soul can only be known from the body. The senses, the best philosophers, teach us that matter is never without form and motion; and whether all matter is sentient or not, certainly all that is sentient is material, and every part of the organism contains a vital principle (the heart of a frog beats for an hour after its removal from the body; the parts of cut-up polyps grow into perfect animals). All ideas come from without, from the senses; without sense-impressions no ideas, without education, few ideas, the mind of a man grown up in isolation remains entirely undeveloped; and since the soul is entirely dependent on the bodily organs, along with which it originates, grows, and declines, it is subject to mortality. Not only animals, as Descartes has shown, but men, who differ from the brutes only in degree, are mere machines; by the soul we mean that part of the body which thinks, and the brain has fine muscles for thinking as the leg its coarse ones for walking. [Footnote 1: La Mettrie was born at St. Malo, and educated in Paris, and in Leyden under Boerhave; he died in Berlin, whither Frederick the Great had called him after he had been driven out of his native land and from Holland. On La Mettrie cf. Lange, _History of Materialism_, vol. ii. pp. 49-91; and DuBois-Reymond's Address, 1875.]