Saint Sorlin and Perrault had argued directly from the permanence of vigour in lions or trees to the permanence of vigour in man. If trees are the same as ever, brains must also be the same. But what about the minor premiss? Who knows that trees are precisely the same? It is an indemonstrable assumption that oaks and beeches in the days of Socrates and Cicero were not slightly better trees than the oaks and beeches of to-day. Fontenelle saw the weakness of this reasoning. He saw that it was necessary to prove that the trees, no less than human brains, have not degenerated. But his a priori proof is simply a statement of the Cartesian principle of the stability of natural processes, which he put in a thoroughly unscientific form. The stability of the laws of nature is a necessary hypothesis, without which science would be impossible. But here it was put to an illegitimate use. For it means that, given precisely the same conditions, the same physical phenomena will occur. Fontenelle therefore was bound to show that conditions had not altered in such a way as to cause changes in the quality of nature's organic productions. He did not do this. He did not take into consideration, for instance, that climatic conditions may vary from age to age as well as from country to country. 4. Having established the natural equality of the Ancients and Moderns, Fontenelle inferred that whatever differences exist are due to external conditions--(1) time; (2) political institutions and the estate of affairs in general. The ancients were prior in time to us, therefore they were the authors of the first inventions. For that, they cannot be regarded as our superiors. If we had been in their place we should have been the inventors, like them; if they were in ours, they would add to those inventions, like us. There is no great mystery in that. We must impute equal merit to the early thinkers who showed the way and to the later thinkers who pursued it. If the ancient attempts to explain the universe have been recently replaced by the discovery of a simple system (the Cartesian), we must consider that the truth could only be reached by the elimination of false routes, and in this way the numbers of the Pythagoreans, the ideas of Plato, the qualities of Aristotle, all served indirectly to advance knowledge. "We are under an obligation to the ancients for having exhausted almost all the false theories that could be formed." Enlightened both by their true views and by their errors, it is not surprising that we should surpass them.