At a time that stressed human development and improvement, Rousseau saw human history in a quite different way. Some of his earliest works, especially his Discourses on the Origins of Inequality, were explicitly historical in nature. They established the idea, as he put it in the opening line of The Social Contract, that "man was born free, but is everywhere in chains." Rousseau used what has been called a "conjectural history," a common device among philosophers who sought to explain human nature.
He imagined a state of nature (another common device) in which people live essentially solitary lives, totally free. In this state, their primary concern is what Rousseau called amour de soi, which basically means self-preservation. Over time, another impulse, which Rousseau called amour-propre, or self-love, caused people to see others as rivals for sexual partners, food, and other essentials.
Rousseau also charted the development of the concept of property, which, combined with the idea of amour-propre, forced people to enter into society, surrendering their freedom in the state of nature for self-preservation. This process, which caused people to imagine themselves as others saw them, rather than on their own terms, was, for Rousseau, the origins of human inequality. This in turn led people to accept their own subjugation, a state which was contrary to humanity itself.
Rousseau, in short, used history as the foundation for his philosophy of morality and of government. This was a common trend in the Enlightenment, but unlike his peers, Rousseau meant for his readers to imagine human development as a gradual departure from our natural selves.