On perfectibility

From Rousseau's Discourse on Inequality (the translation is by Victor Gourevitch):

Why is man alone liable to become imbecile? Is it not that he thus returns to his primitive state and that, whereas the Beast, which has acquired nothing and also has nothing to lose, always keeps its instinct, man again losing through old age or other accidents all that his perfectibility had made him acquire, thus relapses lower than the Beast itself? It would be sad for us to be forced to agree that this distinctive and almost unlimited faculty, is the source of all of man's miseries; that it is the faculty which, by dint of time, draws him out of that original condition in which he would spend tranquil and innocent days; that it is the faculty which, over the centuries, causing his enlightenment and his errors, his vices and his virtues to bloom, eventually makes him his own and Nature's tyrant.