In this essay, Isaiah Berlin traces the path of an old dream of humanity in Occidental thought: the perfect society. These utopian conceptions, all founded on the notion of a fixed and unalterable human nature, can be defined, according to Berlin, as the attempt to restore a broken original unit. In modern times, two visions seem to dispute the field of what is human. There are those who believe that it is possible to attain knowledge that will form the perfect society regardless of the damage and suffering it brings with it. On the other hand, there are those who emphasize the diversity of man, his distinctive qualities and mentalities, open societies, the "living experiments," as used by J.S. Mill. In summary, those who hold that there are no perfect solutions to human affairs. Although it is true that these latter offer a less exultant and heroic course of action, only if this view prevails, Berlin concludes, can men avoid mutual destruction.