21. Alongside of the more strictly literary or humanist movement, too, there went on one of a scientific kind, which divided into two lines, a speculative and a practical. On the former the freelance philosopher Julien Offray la Mettrie gave a powerful initial push by his materialistic theses, in which a medical knowledge that for the time was advanced is applied with a very keen if unsystematic reasoning faculty to the primary problem of mind and body; and others after him continued the impulse. La Mettrie produced his Natural History of the Mind in 1745; [1070] and in 1746 appeared the Essay on the Origin of Human Knowledge of the Abbé Condillac, both essentially rationalistic and anti-theological works, though differing in their psychological positions, Condillac being a non-materialist, though a strong upholder of "sensism." La Mettrie followed up his doctrine with the more definitely materialistic but less heedfully planned works, L'Homme Plante and L'Homme Machine (1748), the second of which, published at Leyden [1071] and wickedly dedicated to the pious Baron von Haller, was burned by order of the magistrates, its author being at the same time expelled from Holland. Both books are remarkable for their originality of thought, biological and ethical. Though La Mettrie professed to think the "greatest degree of probability" was in favour of the existence of a personal God, [1072] his other writings gave small support to the hypothesis; and even in putting it he rejects any inference as to worship. And he goes on to quote very placidly an atheist who insists that only an atheistic world can attain to happiness. It is notable that he, the typical materialist of his age, seems to have been one of its kindliest men, by the consent of all who knew him, [1073] though heedless in his life to the point of ending it by eating a monstrous meal out of bravado. The conventional denunciation of La Mettrie (endorsed by Lord Morley, Voltaire, p. 122) proceeds ostensibly upon those of his writings in which he discussed sexual questions with absolute scientific freedom. He, however, insisted that his theoretic discussion had nothing whatever to do with his practice; and there is no evidence that he lived otherwise than as most men did in his age, and ours. Still, the severe censure passed on him by Diderot (Essai sur les règnes de Claude et de Néron, ed. 1782, ii, 22-24) seems to convict him of at least levity of character. Voltaire several times holds the same tone. But Diderot writes so angrily that his verdict incurs suspicion.