Perfectibility, which is first introduced in Part One of Discourse on Inequality, is first used to distinguish man from the animals. His limitless capacity to develop underlies many of the problems diagnosed in Part Two, however. It is important to grasp the two-sided nature of perfectibility, however. On one hand, it brings man as a species to the limit of his mental and physical capabilities. On the other hand, it is responsible for the misery of individual men because, as well as producing language and reason, it also drives the rise of amour propre and the system of needs that enslave civil man. Without perfectibility, man would still be in the state of nature, and probably a lot happier; however, he would not be human. Any discussion of perfectibility has to consider its role both as the agent of human progress, and as the quality responsible for the many structural faults of modern society.