Man is born free, and everywhere he is in chains. Say what you like about Jean-Jacques Rousseau, but he knew how to write a line. The Social Contract, the political treatise which earned its author exile from his home city of Geneva and a place in the Panthéon in Paris, may not be Rousseau's most entertaining text, nor even his most profound one. But it is the one that did more than any other to inspire the French revolution. Sadly, it also did more than any other to justify the ensuing terror.

While its basic ideas proposed remain interesting, it has long been a point of orthodoxy that The Social Contract is politically impracticable. The principle that the only valid forms of political governance and legislation are those which completely reflect the desire of the population seems absurdly wishful in its thinking. Such uniformity of thought and feeling occurs rarely enough in a single household, let alone across the population of a nation state.

But one keeps coming back to that line at the beginning, which rattles round the reader's head like a wizard's pinball, clocking up points and connections. Its rhetorical force is immense. So much so, in fact, that many have questioned whether it really means anything at all.

The first time I read it, was long before I ever read The Social Contract. It was in an article by Conor Cruise O'Brien in the Heroes and Villains column at the back of the Independent's excellent original Saturday magazine supplement, back in the 1980s.

Rousseau was O'Brien's villain. The Social Contract's opening statement had no more meaning, he suggested, than the parallel idea that "all sheep are born carnivores, and everywhere they eat grass". Man is not born free, was his argument in a nutshell, but is set free by the creation of the human institutions that protect his rights.

The funny thing about this, as I came to realise, is that Rousseau would have agreed. Not about the sheep, but about the fact that it is human institutions that set mankind free. For it is only in one sense that The Social Contract's famous proposition looks forward – that the chains limiting mankind's freedom derive from non-democratic forms of governance, enacting laws which the people neither desire nor approve. But in another sense it looks back to Rousseau's earlier Discourse on the Origins of Inequality, which Rousseau saw as being foundational to the Social Contract's arguments (and indeed to everything he wrote).

It is in this book that Rousseau first unveiled the subsequently much-misunderstood notion of the noble savage. This proto-Darwinian idea that modern man evolved from an animal state was of course deeply shocking to contemporary readers, but it was nothing like as shocking as the idea that savage man in the state of nature is essentially a happier and less depraved creature than the men and women of modern society.

Man in the state of nature is, like animals, equal to his desires in the sense that he does not desire things for which he has no need, or need things for which he has no desire. His consciousness of the world around him, in other words, is efficiently tailored to meeting his needs for survival and reproduction and is not enslaved by the kinds of desire in which today's society specialises: objects and accoutrements whose value exists only in their power to make others see us in a certain light. Modern man, Rousseau argues, is the victim of a divided subjectivity, spreading disorder and unhappiness while convinced that he's acting in his own interests.

This key here is that man in the state of nature lacks individuation and thereby any means to distinguish his individual needs from those of his community. What he does have, however, is what he calls "perfectibilité" – what Darwin would later call adaptability to change. It is through this evolutionary process that human consciousness becomes individuated, and that the sphere of human desire moves beyond what is given to him to desire.

And while it is only at this stage that it becomes proper to speak of human being in the sense (more important in Rousseau's day even than our own) of a morally free being, it is also at precisely this same stage than man becomes depraved, identifying his own interests against and above those of his community, and perceiving desires for things he only needs for increasing his power over others.

The statement that man is born free, and is everywhere in chains, is therefore only partly about politics. On a deeper level it is a statement of a dichotomy fundamental to the idea of mankind (as distinct from the animals): that man's enslavement is the flipside of the coin on which is stamped his basic freedom. Man is free, in other words, precisely because he becomes susceptible to enslavement. And for Rousseau, the one thing that maintains the relationship between the two sides, and prevents enslavement from taking over completely (though he might well argue that it is now too late), is a leftover from our natural state: the supreme human institution of compassion or pity.

The basic idea of The Social Contract is to construct political institutions that allow the rule of compassion to provide the basis for legislation. Although disastrous in practical politics, it is a beautiful idea to which we should pay more than lip service. The idea that we should privilege forms of interaction which develop our compassion – such as, above all for Rousseau, music – remains spot on. If nothing else, his prescience about Darwin's theory of evolution deserves respect. Moreover, sheep are not born carnivorous, whereas man – at least while we still dare to say it – is born free.

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