The wavering gaze.

In her 1977 book On Photography, Susan Sontag suggested that our sympathy for suffering is diminishe

Except for some of her less reflective remarks on current affairs, and her bone-dry, over-reflective novels, it is normally a mistake not to pay attention to what Susan Sontag has to say. As its grimly punning title suggests, this is a book about our responses to images of suffering, and its most interesting feature turns out to be the author's argument with herself. She gently castigates the younger Sontag for her 1977 book On Photography, in which she suggested that our capacity to react emotionally and ethically was being diminished by a vulgar profusion of imagery. "Photographs shrivel sympathy," she concluded. That is not her view today.

She is right not to be too hard on herself - On Photography was a good book - but we shall come to that. Her new work is a somewhat matronly call to order: avoid fancy philosophising about simulacra taking over our lives and keep it real. Horror exists, it stares us in the face (not least through photography), and "right-thinking" people (yes, matron does not shrink from the phrase) should respond appropriately.

What is appropriate is less clear. The problem with images of acute suffering is that they hit you like a hammer: the brain goes dead, but the body still twitches in indignation. A nice example was Virginia Woolf, who, shocked by photographs of battle, said that wars must be stopped "at whatever cost". Whatever? By a pre-emptive strike? Or the surrender of the Bloomsbury way of life? In Sontag's book, explicit moralising is largely absent. Indeed, she warns against the rush of feeling that the spectacle of harsh imagery - famine, war, death - can induce. "Moral indignation, like compassion, cannot dictate a course of action." And though she might have had more to say about the inbuilt deceits of photography, whose anecdotal, non-synthesising nature conflicts with its implicit claim to absolute truth, she recognises that "to photograph is to frame, and to frame is to exclude".

Even when they are as "objective" as can be contrived, photographs offer mere fragments of reality, and when the eyes have it over the intellect, bang goes the big picture. By highlighting what may be unrepresentative detail, the camera obscures. In the words of Roland Barthes (whose brilliant book Camera Lucida: reflections on photography is for some reason not touched on here), a photograph is "the absolute Particular, the sovereign Contingency, matte and somehow stupid".

Then comes the question of veracity. Somehow it is no surprise to discover that almost every iconic image you can think of is in some sense fraudulent. It is quite a thought that love and death in the 20th century are symbolised in millions of minds by a bogus photo-reality. I didn't realise that Robert Doisneau's famous "snap" of a young Parisian couple embracing with enviable elan was staged, and apparently the genuineness of Robert Capa's Death of a Republican Soldier is doubtful (it may record a training exercise). Never mind. Photography is a slippery subject, and at that point you can always argue that this is precisely where it is transmuted into art, since Paris is a truly romantic city, and Franco was a bastard.

Unlike Barthes - who feared that photography, by overlaying other forms of recall, "actually blocks memory, quickly becomes a counter-memory" - Sontag sees it as a powerful and legitimate aid to recollection. At the same time, she agrees there is a risk that "remembering through photographs eclipses other forms of understanding and remembering". Very true, and not just of images of the past. Over-relying on pictures, history and current affairs programmes on television can inhibit our reflective faculties: a shot of a dead US serviceman in Baghdad ("What are they doing there?") eclipses the unpicturesque fact that Saddam Hussein's torture chambers are henceforth empty.

So beguilingly sagacious is Sontag's prose that, for a while, you don't notice a colossal gap in the argument. The First World War, Guernica, the Nazis, the Holocaust, Hiroshima, apartheid, Vietnam, Palestine, Bosnia, genocide in Africa - every 20th-century horror immortalised on film gets a mention. Except communism. Pictures of lynched blacks dangling from trees in America's deep south are hauntingly described and analysed, but nothing evokes the Chinese intellectuals beaten to a pulp by Red Guards, or the officiously murdered kulak.

Sontag would doubtless describe herself as a woman of the left, though she is no Hobsbawm, and so is unlikely to have censored from her gallery of human atrocity the quarter of the world's population who once languished under Stalin and Mao, on the grounds that it was all a worthwhile experiment. My hunch is that hers was a genuine, though enormously revealing, oversight. The more total the dictatorship, the fewer the pictorial records, and communism was more efficient than fascism at discouraging pictures of itself stamping on the human face. Does the availability of imagery skew historical memory? In this book, it would seem so. At the very least, Sontag might have referred us to those propaganda pictures of Russian camps and discussed how these Soviet-style Arbeit Macht Frei institutions took in the Hobsbawms of the period, alluding perhaps in passing to the theory of the late Professor Gombrich that, in all imagery, we see what we want to.

With that major caveat, Sontag's is a mostly reasonable book, though her conclusion about our reaction to harrowing images - that "it is probably not true that people are responding less" - is too tentative to be heartening, and has a whiff of piety. (Like saying that there is no evidence that sexual promiscuity has damaged our capacity for love, it's a consoling thought, even if it is not subject to proof one way or the other.) From Sontag, we expect more to chew on, maybe a piquant paradox or two, so allow me to suggest one: she was right in her first book, and she is right again now.

One purpose in writing this work, I would guess, with its message of "back to the real", is to distance herself from Gallic extravagance; perhaps this is why poor Barthes gets the push, and why Baudrillard is ticked off for suggesting that war has become purely mediatique. Sontag seems anxious to sound a more positive, "sincere" (her word, my inverted commas; it should never be used without them) and sympathetic note. Perhaps she is still under the influence of her distressing experiences in Bosnia, a subject to which she returns several times, naturally enough. Yet even extremely clever people - perhaps especially such people - can be under the illusion that when they have personally seen something, it acquires an extra dimension of reality. Unwitnessed by Sontag, and unfilmed by big-name photographers, the Stalin purges and the cultural revolution also happened.

In any event, I fail to understand her problem with her former, more sceptical self where the camera is concerned. There is no necessary either/or between a healthy cynicism about our overload of imagery and the fresh, receptive eye she calls for. Daily we swing between the two. One moment we surf on simulacra, the next we focus starkly on martyred flesh. Does not this wavering gaze, mesmerised by the banality of images of pain yet intermittently sensing the brute truth behind them, intensify the horror?

It would have been interesting to hear Sontag on Heidegger's striking remark about television: that it does not necessarily bring things closer (in this case human suffering), yet nor does it leave them where they are. TV risks banishing the concept of distance altogether, imposing "a uniformity of non-distance in which everything will be carried away in confusion". It throws everything up in the air, and creates a new uncertainty about what is real (which is not a bad description of where we are, and one that in no way precludes the reality of human pain). The concept could have helped Sontag knit her old and new books together, and although Heidegger is not a man you would normally look to for healing powers, his insight on this occasion could have helped salve what appears to be a distinguished intellectual's conscience about being intellectual.

George Walden is the author of The New Elites: making a career in the masses (Allen Lane, The Penguin Press)