Nevertheless, the idea of progress, so well applied to the evolution of the sciences in the XVII. century, could not then be extended to all social facts. It had met with an insurmountable obstacle in the Middle Ages. Men considered that period as one of retrogression and barbarism, although, as a matter of fact, it was “characterised by the universal perfecting of human sociability.” The idea of progress therefore remained a special one. Thus originated the quarrel of the Ancients and the Moderns[266] whose importance has not been sufficiently understood. The “eminent” Fontenelle and the “judicious” Perrault have very clearly shown in respect to intellectual activity generally considered, what Pascal had already established for science properly so-called.[267] The XVIII. century was full of the idea of progress. But, failing to follow a positive method, it gave a false direction to this idea. It believed in the indefinite perfectibility of man and of society. Now, this notion does not coincide with that of progress. It is even fundamentally opposed to it. Progress signifies “development subject to fixed conditions, and operating in virtue of necessary laws, which determine its advance and its limitations.” It is precisely the ignorance of these conditions and of these laws which gives rise to the idea of indefinite perfectibility. If Helvetius and Condorcet had had a positive knowledge of human nature, they would not have entertained so many illusions and unreasonable hopes. Biology, that is to say, scientific psychology, would have taught them that human nature is invariable in its basis, that the preponderance of the selfish over the altruistic instincts is essential to this nature, and that, if progress favours the development of the altruistic feelings, it cannot, however, overturn the natural equilibrium of our inclinations. In a word, indefinite perfectibility is a metaphysical idea. Imagination plays a greater part in it than observation. The philosophers who conceived it did not realise the relations which bind the intellectual and the moral life of man to the structure of his organism.