It is rare that the death of a stranger brings a stab of mourning, but I felt one when I heard on Saturday night that the American philosopher Richard Rorty had succumbed to cancer at the age of 75. At the time of his passing Rorty was the single most important living American philosopher and one of the most influential and widely read thinkers in the English-speaking world. He was also the model of a truly engaged public intellectual, writing with verve, humor and insight for a general audience in magazines like the Nation and Dissent. The world in general and the global community of those fighting for a more just, humane and social democratic world are poorer for his loss.

Rorty made his first, and perhaps most famous, major contribution to philosophy in 1979 with the publication of Philosophy and the Mirror of Nature. The book offered a bracing and eloquent critique of the western philosophical tradition, which holds that there is an absolute reality independent of the human mind and that the job of those who seek knowledge is to "mirror" this reality. No, said Rorty. There's just us - just humans grasping in the darkness, making meaning through argument and consensus, with no Platonic bedrock "out there" to sink our anchors into. In marching back the grandiose truth claims of much of western philosophy, Rorty succeeded in reviving the dormant tradition of pragmatism, an embedded, practical, distinctly American philosophical school pioneered by William James and Charles Peirce and further developed by one of Rorty's great heroes, John Dewey.

After epistemology, Rorty turned his thoughts to moral and political philosophy, where he wrestled with the fundamental dilemma of post-modernity: if there is no divine law, no universal, capital-T Truth, then how to avoid nihilism and relativism? Is it true, as Ivan Karimazov said, that "If God does not exist, then everything is permitted?"

Fundamentalists, from Pennsylvania avenue to Pakistan, say yes. Without the absolute, unwavering divine law, they argue, there is chaos and cruelty. We cannot deny God, or Allah, or the Infinite Justice of America's War on Terror, and retain any semblance of moral order. But Rorty disagreed with eloquence and force. Yes, he, said, there is no God, but that doesn't mean everything is permitted: we need not descend into cruelty and inhumanity. We can, as humans with our limited faculties, hash things out amongst ourselves, and arrive at a just order, one that minimizes cruelty and suffering, and engenders equality and solidarity. How to go about that? No easy answers, said Rorty. "There is no basis for deciding what counts as knowledge and truth other than what one's peers will let one get away with in the open exchange of claims, counterclaims and reasons."

For his critics, this kind of pragmatist shrug could be maddening, but the intellectual humility it embodied seems wise beyond measure in the post-9/11 age. In the wake of that tragedy commentators declared the "death of irony", and the Bush administration, with much of the nation's support, rushed into a war convinced utterly of its own righteousness - convinced that there is a Good and Evil out there, that we are the former, and our enemies the latter. Perhaps it was Rorty's own highly developed ironic sense that led him to vociferously oppose the war, arguing against it in a 2003 cover story for the Nation. For much of his life, Rorty called himself a "liberal ironist", which is, in every way, the direct opposite of Bush, who is nothing if not an earnest reactionary.

I had the good fortune to meet Richard Rorty once, several years ago in Chicago. He was in town to debate his old friend and sparring partner, the German philosopher Jurgen Habermas. In person, the two men were quite different from their personas on the page - Habermas was jocular and charming; Rorty, droll and reserved. He seemed to view the world before him with an air of bemusement, and it occurred to me that his ironic sensibility was as much disposition as ideology.

When Rorty addressed the packed hall assembled for his debate with Habermas, he swung his leg at the podium and impishly pushed the crowd's buttons. "We think of ourselves as having made progress as we grow older, more wise, less capable of doing harm," he said. "Maybe we're right and maybe we're wrong."

During the Q and A that followed, Rorty casually mentioned that it was "true" that "we are using too many of the world's resources too quickly and irresponsibly." Someone shot up their hand to ask him what exactly he meant by the word "true" in that sentence.

He responded: "In a free and open forum everyone would agree we're using too many resources."

"But not everyone does agree!" someone shouted from the audience in exasperation.

"I know," Rorty responded dryly. "I'm still trying to figure that out."

So, too, are we. And it will be a much, much harder task without him.