The aim of the Discourse is to examine the foundations of inequality among men, and to determine whether this inequality is authorized by natural law. Rousseau attempts to demonstrate that modern moral inequality, which is created by an agreement between men, is unnatural and unrelated to the true nature of man. To examine natural law, Rousseau argues, it is necessary to consider human nature and to chart how that nature has evolved over the centuries to produce modern man and modern society.
To do this, he begins in the imaginary state of nature, a condition before society and the development of reason. Discarding the Biblical account of human creation and development, Rousseau attempts to conjecture, or guess, what man in this state would be like. He examines man's physical and mental characteristics, and finds him to be an animal like any other, motivated by two key principles: pity and self-preservation. The only real attribute that separates him from the animals is his perfectibility, a quality that is vitally important in the process Rousseau goes on to describe. Man in the state of nature has few needs, no idea of good and evil, and little contact with other humans. Nevertheless, he is happy.
However, man does not remain unchanged. The quality of perfectibility allows him to be shaped by, and to change in response to, his environment. Natural forces such as earthquakes and floods drive men into all parts of the globe, and force them to develop language and other skills. As men come into contact more frequently, small groups or societies start to form. The human mind begins to develop, and as man becomes more aware of others, he develops a series of new needs. The emergence of reason and society are related, but the process by which they evolve is a negative one. As men start to live in groups, pity and self- preservation are replaced by amour propre, which drives men to compare themselves to others, and to need to dominate others in order to be happy.
The invention of property and the division of labor represent the beginning of moral inequality. Property allows for the domination and exploitation of the poor by the rich. Initially, however, relations between rich and poor are dangerous and unstable, leading to a violent state of war. As an attempt to escape from this war, the rich trick the poor into creating a political society. The poor believe that this creation will secure their freedom and safety, but in fact it merely fixes the relations of domination that existed before, creating laws to establish inequality. Inequality is now more or less unrelated to man's original nature; physical inequality is replaced by moral inequality.
Rousseau's account of the operation of society focuses on its various stages. Beginning with the trick played by the rich, he sees society as becoming more and more unequal, until its last stage, which is despotism, or the unjust rule of everyone by one man. This development is not inevitable, but it is extremely likely. As wealth becomes the standard by which men are compared, conflict and despotism become possible. For Rousseau, the worst kind of modern society is that in which money is the only measure of value.
Rousseau's conclusions to the Discourse are clear: inequality is natural only when it relates to physical differences between men. In modern societies, however, inequality derives from a process of human evolution that has corrupted man's nature and subjected him to laws and property, both of which support a new, unjustifiable kind of inequality, termed moral inequality. This is an unacceptable situation, according to Rousseau, but he gives few clues about how it can be improved.