We saw that the "solidarity" of the sciences was one of the conceptions associated with the theory of intellectual progress, and that the popularisation of knowledge was another. Both these conceptions inspired the Encyclopaedia, which was to gather up and concentrate the illumination of the modern age. It was to establish the lines of communication among all departments, "to enclose in the unity of a system the infinitely various branches of knowledge." And it was to be a library of popular instruction. But it was also intended to be an organ of propaganda. In the history of the intellectual revolution it is in some ways the successor of the Dictionary of Bayle, which, two generations before, collected the material of war to demolish traditional doctrines. The Encyclopaedia carried on the campaign against authority and superstition by indirect methods, but it was the work of men who were not sceptics like Bayle, but had ideals, positive purposes, and social hopes. They were not only confident in reason and in science, but most of them had also a more or less definite belief in the possibility of an advance of humanity towards perfection. As one of their own band afterwards remarked, they were less occupied in enlarging the bounds of knowledge than in spreading the light and making war on prejudice. [Footnote: Condorcet, Esquisse, p. 206 (ed. 1822).] The views of the individual contributors differed greatly, and they cannot be called a school, but they agreed so far in common tendencies that they were able to form a co-operative alliance. The propaganda of which the Encyclopaedia was the centre was reinforced by the independent publications of some of the leading men who collaborated or were closely connected with their circle, notably those of Diderot himself, Baron d'Holbach, and Helvetius. 3. The optimism of the Encyclopaedists was really based on an intense consciousness of the enlightenment of their own age. The progressiveness of knowledge was taken as axiomatic, but was there any guarantee that the light, now confined to small circles, could ever enlighten the world and regenerate mankind? They found the guarantee they required, not in an induction from the past experience of the race, but in an a priori theory: the indefinite malleability of human nature by education and institutions. This had been, as we saw, assumed by the Abbe de Saint-Pierre. It pervaded the speculation of the age, and was formally deduced from the sensational psychology of Locke and Condillac. It was developed, in an extreme form, in the work of Helvetius, De l'esprit (1758).