Few thinkers have left their fingerprints on the modern age as indelibly as Jean-Jacques Rousseau, the tricentenary of whose birth we celebrate on Thursday (28 June).. He was a philosopher who helped shape the destiny of nations, which is more than can be said for Pythagoras or AC Grayling. He was also a political visionary of stunning originality, a potent influence on the French revolution and a source of inspiration for the Romantics. Those who like their fiction drenched in lofty moral sentiment can also claim him as a great novelist.

Much of what one might call the modern sensibility was this thinker's creation. It is in Rousseau's writing above all that history begins to turn from upper-class honour to middle-class humanitarianism. Pity, sympathy and compassion lie at the centre of his moral vision. Values associated with the feminine begin to infiltrate social existence as a whole, rather than being confined to the domestic sphere. Gentlemen begin to weep in public, while children are viewed as human beings in their own right rather than defective adults.

Above all, Rousseau is the explorer of that dark continent, the modern self. It is no surprise that he wrote one of the most magnificent autobiographies of all time, his Confessions. Personal experience starts to take on a significance it never had for Plato or Descartes. What matters now is less objective truth than truth-to-self – a passionate conviction that one's identity is uniquely precious, and that expressing it as freely and richly as possible is a sacred duty. In this belief, Rousseau is a forerunner not only of the Romantics, but of the liberals, existentialists and spiritual individualists of modern times.

It is true that he seems to have held the view that no identity was more uniquely precious than his own. For all his cult of tenderness and affection, Rousseau was not the kind of man with whom one would share one's picnic. He was the worst kind of hypochondriac – one who really is always ill – and that most dangerous of paranoiacs – one who really is persecuted. Even so, at the heart of an 18th-century Enlightenment devoted to reason and civilisation, this maverick intellectual spoke up for sentiment and nature. He was not, to be sure, as besotted by the notion of the noble savage as some have considered. But he was certainly a scourge of the idea of civilisation, which struck him for the most part as exploitative and corrupt.

In this, he was a notable precursor of Karl Marx. Private property, he wrote, brings war, poverty and class conflict in its wake. It converts "clever usurpation into inalienable right". Most social order is a fraud perpetrated by the rich on the poor to protect their privileges. The law, he considered, generally backs the strong over the weak; justice is largely a weapon of violence and domination, while culture, science, the arts and religion are harnessed to the task of preserving the status quo. The institution of the state has "bound new fetters on the poor and given new powers to the rich". For the benefit of a few ambitious men, he comments, "the human race has been subjected to labour, servitude and misery".

He was not, as it happens, opposed to private property as such. His outlook was that of the petty-bourgeois peasant, clinging to his hard-won independence in the face of power and privilege. He sometimes writes as though any form of dependence on others is despicable. Yet he was a radical egalitarian in an age when such thinkers were hard to find. Almost uniquely for his age, he also believed in the absolute sovereignty of the people. To bow to a law one did not have a hand in creating was a recipe for tyranny. Self-determination lay at the root of all ethics and politics. Human beings might misuse their freedom, but they were not truly human without it.

What would this giant of Geneva have thought of Europe 300 years on from his birth? He would no doubt have been appalled by the drastic shrinking of the public sphere. His greatest work, The Social Contract, speaks up for the rights of the citizenry in the teeth of private interests. He would also be struck by the way the democracy he cherished so dearly is under siege from corporate power and a manipulative media. Society, he taught, was a matter of common bonds, not just a commercial transaction. In true republican fashion, it was a place where men and women could flourish as ends in themselves, not as a set of devices for promoting their selfish interests.

The same, he thought, should be true of education. Rousseau ranks among the great educational theorists of the modern era, even if he was the last man to put in charge of a classroom. Young adults, he thought, should be allowed to develop their capabilities in their distinctive way. They should also delight in doing so as an end in itself. In the higher education systems of today's world, this outlandish idea is almost dead on its feet. It is nearly as alien as the notion that the purpose of education is to serve the empire. Universities are no longer educational in any sense of the word that Rousseau would have recognised. Instead, they have become unabashed instruments of capital. Confronted with this squalid betrayal, one imagines he would have felt sick and oppressed. As, indeed, he usually did.

• Follow Comment is free on Twitter @commentisfree