Rousseau begins by twisting the prize question towards his own particular agenda. The original question concerns what is the nature of inequality among men, and whether it is authorized by the natural law. Rousseau asks another, related question: how can one know inequality without knowing man? To answer this question we must not consider man as he is now, deformed by society, but as he was in nature. Progress drives man as a species further from its original condition in the state of nature. As knowledge increases, so our ignorance of the true nature of man increases.
Rousseau acknowledges the hypothetical and conjectural nature of what he is about to do in the Discourse. Undertaking to disentangle the natural from the artificial in man is a difficult task indeed. What is needed is a kind of experiment to achieve this. At the moment, ignorance of the nature of man casts uncertainty over the nature of natural right. Rousseau provides a brief account of the ancient and modern debate over natural rights and natural law.
A second problem arises; if we are uncertain about what the terms nature and law mean, how can we define the natural law that is supposed to authorize inequality? In considering this question, we return to the problem of the real nature of man. For if we are ignorant of man's nature, it is impossible to tell whether the definition of natural law we decide on fits with that nature at all. To be a law, it has to be agreed to "knowingly" (rationally), and to be natural it must "speak with the voice of nature."
There is a way out of this problem, however. Rousseau next claims that he perceives two basic principles that exist "prior to reason"—that is, before man is deformed by society and rationality. These are self-preservation and pity. From these principles, which do not require sociability, natural right flows. Man's duties are not dictated to him by reason alone, but by self-preservation and pity. Therefore a man will not harm another sentient (pain-feeling) being unless his own self-preservation is at stake. The duty not to harm others is based not on rationality but on sentience, the state of being able to feel. According to Rousseau, this solves the age-old question of whether animals participate in natural law. As they are not rational, he says, animals cannot have any part in a natural law, but as sentient beings they take part in natural right, that is, they feel and are the subjects of pity. This gives animals at least the right not to be mistreated by man.
The study of natural man, of his "true needs" and "fundamental principles of his duty," is the only way to clear up important issues such as the origin of moral inequality and the foundations of the "body politic" (the state). Without such a study, the foundations of modern society seem shaky and insecure, and it is hard to separate what "divine will" intended from what man himself created. By realizing what we would have been if left to ourselves, Rousseau argues that we can better appreciate "him whose beneficient hand" steered us away from the worst disorders.
The Preface was probably written for the published version of the Discourse, and is essentially Rousseau's attempt to define the problem he is about to tackle. He makes his methodology and assumptions clear at the beginning of the work, and shows some of the problems with the terms he uses. His first move is an important one: shifting the focus of the question towards the nature of man gives the Discourse added depth. All questions about inequality and modern society depend on one question: what is natural?