All human beings deserve an equality of respect, of opportunity. This I know to be true, and it is toward the goal of securing this, in a radically unequal world, that I work. But the notion of a Truth, absolute and secure, which underpins my work and that of all of us who work in human rights, is deeply suspect philosophically, particularly in this age of cultural relativism. To evoke its moral magic, human rights needs to exude a special kind of certainty, of old-fashioned clarity, but this can mean that those of us who use the term can appear dated, even smug - we know the right answers; we have special access to the truth. While, in the legal and political sphere, human rights is undoubtedly a successful concept - enshrined in laws across the world - the apparently anachronistic notion of truth at its core - embedded in concepts like 'unalienable rights' - could be it's undoing. Though it may not look like it now, the concept of 'human rights' faces a crisis of authority.

Political philosopher Jurgen Habermas has remarked of religious beliefs that they require 'striking cognitive dissonances' since, as he puts it, "the complex life circumstances in modern pluralistic societies are normatively compatible only with a strict universalism in which the same respect is demanded for everybody - be they Catholic, Protestant, Muslim, Jewish, Hindu, or Buddhist, believers or non-believers." So their various 'truths' are self-evidently in difficulty, undermined by the very conditions of their continuance.

But, religious followers have it easy: after all, many religious folk are not in 'modern pluralistic societies', and so can remain undisturbed by Habermas's paradox. But even if they are, they have many bulwarks against crisis - the confidence of a way of life that has been around for centuries; the support of a community of believers; the leadership of charismatic figures; above all confidence in some kind of god or spirit that speaks directly to their situation, both individually and collectively. For many, of course, there is the additional consolation of an afterlife.

By comparison, the human rights believer is lonely and vulnerable. They seek Heaven on earth for all, not just (or even mainly) for themselves or the chosen few. And there is no pre-modern refuge: it is firmly and only within 'the complex life circumstances in modern pluralistic societies' that they must ply their trade. The genuine human rights believer cannot thrive outside pluralism. But pluralism's shelter for human rights leaks with doubt: in a place where everything is true, nothing can be really true. Human rights people are stuck, required without the support of many potent symbols to practise their beliefs in exactly those places - developed modern societies - where belief in anything is hard enough and belief in something moral, apparently rooted in human nature, hardest of all. They are the disciples of an idea rather than a holy text or being and the closest they get to congregational worship is the occasional drinks party after a human rights lecture. (They are usually too polite for the solidarity that comes from public protest.)

If the idea of human rights amounts to, as Francesca Klug has put it, a set of 'values for a Godless age', then the custodians of these values face a tough task. For what are the philosophical foundations of human rights? The growth of the term in political and legal spheres has been an unambiguous success - the plethora of international, regional and legal instruments that embed the term in various codes of law has been one of the most remarkable features of legal development in the past 60 years. Similarly much political activity is suffused with the language of rights; it is as though not to assert a right to what one desires is to make a fatal admission of its unimportance. Yet this success in law and politics has been grimly paralleled by its philosophical decline. As a shorthand description of how we want the world to improve, 'human rights' has ethical cache and works very well. But it is not guaranteed to continue to do this without viable philosophical undergirding.

Without a reworking of what the term means today, designed to give it contemporary intellectual confidence, some theoretical zest, then the time might come when firing the human rights argument will be greeted neither with warmth nor dismay but rather with blank indifference, or mute incomprehension: "er, what did that used to mean?" Or, perhaps, official doublespeak: "beating that guy up in that way is compatible with his human rights, and I have the judicial warrant to prove it."

Without a secure, philosophical, idea of what 'human rights' might mean, the dangerous side-effects of its victory could outweigh the benefits: the triumph of legal form over substance, the subjugation of the emancipatory potential of human rights to the interests of the already powerful.

The challenge for those who want to see the idea of human rights survive, indeed continue to thrive, is to close the gap between its political and legal success and its philosophical (or should we say post-philosophical) shortcomings. To do this, it is necessary to reconstruct the foundations for human rights, to build support systems that defend the term not by the simple invocation of the past glories of a 'golden age' but in terms that ring true today, that allow us to name abuses of human rights when we see them, even those done in the name of human rights. This task is made tougher by the unpalatable realisation that it is very hard indeed to pin down when exactly the supposed golden age of human rights actually was.

The subject has never been lucky in its intellectual apologists; its heroes are not good at transcending their particular moment and speaking clearly to us today. The Greeks knew all about natural justice and fairness, but did not have the words to describe the notion of a subjective right invested in persons on simple account of their humanity. The Roman church embraced the concept of natural law with enthusiasm, but for her a right was located in the air, not in the bodies walking the earth beneath. The Protestant concept of individuality introduced a notion of subjectivity, through which the idea of individual rights could emerge, but their manifestly Christian basis made them an uncertain ally when our contemporary, very secular, culture began to emerge. The field was clear then for the emergence of the great intellectual leaders of the human rights movement, fantasists of human nature all - Hobbes, Locke, Kant, Rousseau. Their contribution was profound, but they seem more and more to belong to their historical moment than to be the deliverers of transcendental truth which they were once hailed. Ideas about 'states of nature', 'noble savages' and the like seem contrived and old-fashioned now, arguing for a truth that even at their pinnacle was never more than hypothetical.

After a decline of human rights speak in the 19th century, comes the fundamental challenge of Darwin. Up to this point human rights, philosophically, resided in the mind, as an idea not a feeling, as a transcendental value. Darwin blows apart the Cartesian mind/body dualism which could imagine human rights residing in the separate and inherently superior domain of the mind. Darwin, according to TC Grey "made it plausible to treat human mental capabilities as evolved functions of natural organisms": he took our minds out of some unique spiritual ether and put them firmly back in our bodies.

After Darwin it became clear that we were part of our habitat, 'clever animals' as Nietzsche said, differing from the rest only in our greater capacity to redescribe and therefore recreate ourselves. There is no room here for any truth, let alone Truth, apart from that which underpins the structure of the words with which we describe what we call the world around us. But this kind of second order lower case truth is insufficient to make the obligations inherent in the idea of human rights work effectively today: it won't allow us to name right and wrong, let alone punish the bad guys.

In such a world of uncertainty, where can we find the core ideas with which to reconstruct our subject? We can start by embracing Darwin and accepting the contingency of language, two forces destructive to the old model of human rights. Darwin offers us a way to think about what it is that joins us to all living creatures, and what makes us special. The contingency of language requires us to call off our impossible search for immutable Truth through words, replacing it instead with a different but more explicitly humanist project: one that pragmatically seeks to turn our words to the doing of good work for our species, namely the reduction of cruelty and the enabling of all of us to lead successful lives.

A post-Darwin human rights needs to ask, what are the special features that this 'clever animal' the human has, over and above the other animals with which it shares the world? Three in particular come to mind: first, this animal is self-conscious, capable of critical reflection of itself and where it fits in the environment in which it finds itself. Second, it is aware of death, both generally and specifically with regard to its own individual self. Third, this animal is capable of a set of contradictory impulses the import of which, because it is a self-conscious being, it understands: on the one hand there is the capacity for acts of compassion, hospitality and kindliness, on the other for cruelty, humiliation and callousness.

Looked at from the victim's point of view, the consequence of the latter is indeed shared with the animals; the ability to feel pain is not linguistic in a narrow sense of the term: it requires no words to make it the case. The doyen of liberal ironists, philosopher Richard Rorty has written that "there is something within human beings which deserves respect and protection quite independently of the language they speak. It suggests a non-linguistic ability, the ability to feel pain, is what is important, and that differences in vocabulary are much less important." This is true, but does it follow that because pain is non-linguistic it can have no language? Rorty certainly thinks so: "victims of cruelty, people who are suffering, do not have much in the way of a language. There is no such thing as the 'voice of the oppressed' or 'the language of the victims'."

I disagree. There is such a language: The language of human rights. A language that speaks for people and that manages, by forcing people to be visible to everyone, to enable, first, people to speak for them and, then, for them to speak for themselves. It is a language that is genuinely universal; cruelty is as real in Harare as it is in Hampstead. It is the language of hospitality and kindliness, but above all of compassion. Here is a word - compassion - that best captures the kind of active concern for others that the term 'human rights' should signify. Compassion, as the theologian Oliver Davies has argued, is neither love nor mercy; it has a cognitive element (understanding the other), an affective element (feeling for the other) and a voluntarist element (doing something about the other). Davies observes that there "is something subversive, indiscriminate and boundary-crossing about compassion" and he is right about this - it is through the rallying power of compassion that we can use human rights to frame and mobilise responses to suffering and to atrocities. Compassion is a universalistic disposition from which virtue flows and the linguistic medium through which it expresses itself is the language of human rights, the Esperanto of the virtuous.

The 'unconditional hospitality' (Derrida's phrase) that flows from compassion is wider, richer and more active than mere tolerance; it is about enjoying and enabling the other to thrive rather than simply putting up with them. This is more than mere forbearance from cruelty and humiliation, important though such conscious restraint is. This human rights language asserts that we are all equal in view of our humanity and that our dignity, rooted in wonder at the brute fact of our achievement, demands that we, each of us, be given the chance to do the best we can, to flourish, to do something with ourselves. Remembering Darwin we must note that this is not necessarily a religious insight: as the Jesuit scholar Jack Mahoney writes in a forthcoming book on human rights, "the emergence of the human being even as a matter of chance in a blind cosmos is a genuine cause for wonder; and the product of the human species in the course of evolution is something to be wondered at with something approaching awe."

Do we need the language of human rights for all this? Theoretically no: With the liberal ironists we might say that "recognition of a common susceptibility to humiliation is the only social bond that is needed" and that our "sense of human solidarity is based on a sense of common danger, not a common possession or a shared power." According to Richard Rorty, "human rights are superstitions - contrivances put forward by the weak to protect themselves against the strong", and the notion of 'inalienable human rights' merely a "slogan no more or less true than 'obedience to God'".

But in what language would Rorty have us defend the basis of our liberal society he holds so dear? "A liberal society is one whose ideals can be fulfilled by persuasion rather than force, by reform rather than revolution, by the free and open encounters of present linguistic and other practices with suggestions for new practices." It is clear today that this kind of liberal society cannot be taken for granted. It is under attack from various enemies: economic power, fundamentalist religion, national exclusivity among them. The flip-side of liberalism, the commitment to compassion over cruelty and to personal flourishing over public prejudice, is also more exposed than it has been since the pluralist view of the world first grabbed our imagination and persuaded us of its truth. Our democratic and legal processes are already in severe danger of being captured by the rich, while our public culture is increasingly filled with the noise of demonising rabble rousers. With what can liberal society fight back?

Religious words don't work anymore, and talk of rationally based moral obligation is also beside the point. But the liberal ironist wants us also the change the subject when human rights come up.

In this current age of doubt, with cruelty rampant in the gaps left in our culture by the abandonment of all our truths, we have reached the point where we should now admit that, contra Eliot, it is with too much unreality that human kind finds it hard to cope. We need truths even if we have to make them up.

It is not enough to leave everything to sentiment. Our culture is simply not up to jettisoning so much of the past while holding out such intangible and unsupported hope for the future. And if the good guys give up on the language of human rights, then others - less principled; differently motivated - will fill the words with a bleaker kind of meaning, ridiculing their preposterous breadth perhaps, or using the term to justify killing foreigners with differently coloured skins. The term 'human rights' is the phrase we use when trying to describe decency in our post-philosophical age. The phrase will not disappear if over-scrupulous liberals refuse to have anything to do with it: the words exist, they can be made to do good work: the true pragmatist would embrace the idea and make it work, not wander from the battlefield of meaning with intellectual purity intact but honour in shreds.

Conor Gearty is Rausing Director of Centre for the Study of Human Rights and Professor of Human Rights Law, LSE. His book Can Human Rights Survive? was published by Cambridge University Press in Spring 2006.