`I recorded it all as history unfolding, believing that it could never happen again'

Biafra, Bangladesh, Vietnam: name any of the world's trajedies of the past 20 years, and likely as not Don McCullin broke the news. But what are we to make of these images when taken off the front page and pinned up in a gallery?

Don McCullin's pictures are not what they were. Take the picture on this page. When it first appeared in print, it was an immediate bulletin from the world, an urgent appeal crying out of its emergency, designed to appal and to baffle the conscience of its original viewers. And today it's different. It's not news, it's art. Not, of course, that such a photo couldn't be taken somewhere in the contemporary world, but this one wasn't. It's a "classic" image, a Don McCullin picture, an example of an uvre. It urges nothing. It's become partly a historical document, and partly a fixed emblem, reflecting a timeless and universal theme: human suffering.

There are about 200 pictures in the Barbican's McCullin retrospective, "Sleeping with Ghosts". The majority represent his 20 years working as a photojournalist, from the mid-Sixties to the mid-Eighties, on assignments to the world's disaster areas. Cyprus, Biafra, Londonderry, Bangladesh, Vietnam, Cambodia, Beirut: this is McCullin's roster, and the exhibition's, all those place-names that have become general names for crisis and tragedy. And all McCullin's most powerful shots are here: the Cypriot widow caught in a convulsion of grief, the young Biafran mother with a baby sucking her withered breast, the stick-legged Biafran albino boy, the dumb shell-shocked GI, the abandoned Bangladeshi baby crying in the road - the images which have put a memorable face to suffering for this generation.

But this seems the wrong way of putting it, too much like art. Praising the pictures' power and memorability, don't we treat the subject as if it were only a contribution to that power? Don't we risk substituting the photographer's vision, the photographer's "face", for the faces he witnessed? Shouldn't the claims of the subject come first? The distinction isn't so easy to make. It was McCullin's camera, his professional vocation and his personal courage that helped to make those names. And if his own name became in its time a by-word, his presence a guarantee, of a situation's urgency and gravity, then McCullin very largely created his subject. Few photographers before him had brought this news so forcefully and immediately. The proper question is: what kind of news was it, and is it?

Looking back, McCullin himself has drawn a desperate lesson: "I recorded it all as history unfolding, believing that this could never happen again, but it has and it will. I am under no illusions now. Humanity will go on suffering to the end of time." The statement is quoted on a large wall- caption in the exhibition, and thus given a kind of summary authority. And it's true that, when seen all together, in retrospect, in an art gallery, this is the lesson that the pictures tend to bear. As you go round the show, from one catastrophe to another, human suffering looks like something that keeps on happening. Each picture is one more updating verification of this permanent, proverbial truth. That's its "news".

At the same time, this lesson must seem too proverbial, too general, too accepting, too near to the churchman who's ever ready with a topical crisis prayer ("We think especially at this time of the people of Bosnia and Rwanda...") and never daunted by the way each year the dotted line can be filled with some new name. The kind of suffering McCullin bore witness to, it wasn't just suffering, it had its causes; it wasn't just humanity either, it was particular sections of humanity. And even if humanity will go on suffering to the end of time, that hardly diminishes an obligation to relieve it, prevent it, question it, protest against it. A sense of that obligation was surely what the pictures were first designed to awaken and enforce.

Not entirely. There is a split here, which runs through McCullin's work. His images are torn between two modes of address - between shock and contemplation. Nor is this merely the work of time or the effect of seeing them in a gallery (reportage becoming art, the contemporary passing). The photos had this "contemplative" aspect from the start: at odds with their urgent appeals, but a function of them too.

The paradox is almost inherent. To communicate suffering most directly, one must put a face to it, concentrate on immediate victims; and thus concentrate on suffering as such. McCullin's vision is close-up, in the midst of things, on individuals. But not exactly on individuals: the agony envelops everything. There's little to indicate lives beyond the immediate circumstances or the abnormality of these circumstances. His subjects are identified with their fate. His strongest pictures use a kind of allegorical portraiture. The subjects acknowledge the camera, stand centred, half pose for it. This alleviates any feeling of voyeurism, of images stolen. It confers a dignity on degradation. But it also makes the people seem to present and perform their suffering for us, to become its living symbols, personifications of hunger, grief, violation, endurance.

In the chaos of misery and conflict, McCullin finds modern Pietas, Martyrs showing their wounds, Dead Christs. He holds them in compositions of great solidity and solemnity, which establish their instants as fixed tableaux. He prints them in grave, dark textures that fuse the sight witnessed on to the picture surface. And all these devices put out of mind the chanciness and the partialness of the camera's relation to the world, a world whose causes and circumstances aren't and often can't be photographed. They make the camera's glimpses look like they were always meant to be images, and they make the images look complete in themselves.

This is what gives his photos their lasting power, but it's also what gave them their immediate effect. They appealed through speaking a permanent language of torment and pathos. Or rather: they filled old pictures with new realities, gave them the jolt of news. But the forms remain religious, the types of a world-view in which suffering has a place.

So, as you go round this exhibition, you fall into a steady attitude that seems quite natural. Not indifferent, not flinching, not exactly consuming it, but not protesting against it either; above all, not wishing it otherwise: you simply contemplate it, trying to absorb it, to take its measure, as something well-known but right to be dwelt on. It is a kind of wonder - and of course there's a way in which these horrors and catastrophes can become world-wonders - with, all the while, a troubling consciousness that this attitude is deeply wrong.

There isn't an obvious means of resolving this split perspective. It seems pointless to accuse McCullin of "aestheticising" suffering, or making a "spectacle" of it. True, even within the limits of what photography can show about the world, he might have worked differently, made images that looked less inevitable. And true, there does seem something mad about us viewers giving our solemn attention to a gallery of past horrors which by definition we can do nothing about, when present horrors might equally fill these walls. What should we say: that McCullin's work, having done its job, or not done its job, ought simply to be put away? That keeping it on view is actually dangerous, because it can now only dull the conscience, by making such outrages seem familiar and expected, by establishing a genre of image-and-response that all new photographs of new outrages will settle into in their turn?

Perhaps all true. But McCullin's pictures tell a truth too. Not necessarily that humanity will suffer to the end of time, but that some kinds of suffering, in their extremity and absoluteness, can only be known and contemplated - and that in this sense there's nothing to be done about them. It is a quasi-religious truth, and there's nothing to be done with it either; not the whole truth, a truth very liable to comfortable corruption, a truth which the arts are all too good at telling. But still, McCullin's work should be praised for it - though not too loudly or eagerly.

To 14 Dec, Barbican Centre, London W2 (0171-382 7105)