Bouts, Dieric: Martyrdom of St Erasmus - Triptych (c1458)

"About suffering they were never wrong/ The Old Masters," W H Auden said, in his poem "Musée des Beaux Arts". No, they were very realistic, he thought. They always reminded us that catastrophe co-exists with the everyday. Even while, for some, life is ending horribly, for others it goes indifferently on. Icarus falls out of the sky. At the same time, a man calmly ploughs a field. Auden was mainly thinking of Brueghel.

"About suffering they were never wrong/ The Old Masters," W H Auden said, in his poem "Musée des Beaux Arts". No, they were very realistic, he thought. They always reminded us that catastrophe co-exists with the everyday. Even while, for some, life is ending horribly, for others it goes indifferently on. Icarus falls out of the sky. At the same time, a man calmly ploughs a field. Auden was mainly thinking of Brueghel.

But when it comes to suffering and indifference, painting can seem far from realistic. What about those martyrdoms, with their huge under-reactions? It's not just the bystanders, the victims are calm too. In his play, A Question of Attribution, Alan Bennett puts a speech about this into the mouth of his protagonist, Anthony Blunt, art historian and traitor. Blunt is giving a slide lecture. On the screen behind him, the images come up, scenes of saints being tortured to death, "howling agonies gone through without a murmur". And Blunt remarks, (with a twist on Auden): "Remote though this is from our sensibility, there is a sense in which one might feel it is all very British. For flayed, dismembered, spitted, roasted, these martyrs seldom lose a drop of their sang-froid, so cool about their bizarre torments, the real emblems of their martyrdom a silk dressing gown and a long cigarette-holder' all of them doing their far, far better thing in a dignified silence. About suffering they were always wrong, the Old Masters."

Now the speech is partly a joke on Blunt's own affectless insouciance - showing the scholar's prim bewilderment in the face of extreme violence, and the traitor's detachment from human consequences. But it's also a joke about art, about the strangeness of art. Through this speech, Bennett voices a puzzlement felt about martyrs in pictures. As Philip Larkin asked in another context: Why aren't they screaming? Bennett's stiff-upper-lip idea is only a joke answer to that question - a way of pointing out how odd these pictures look. Perhaps the real answer is that the pictures got it wrong. "The Old Masters" didn't know what suffering looked like, or how to depict it. But that can't be the reason. Think of all those pictures of the damned, writhing and yelling. They could do it if they wanted to.

So perhaps it's us who are wrong. We shouldn't take the images so literally. They're not meant to be realistic. They're deliberately avoiding explicit horror, observing decorum. It's like an old Western, with its blazing gunfights, where all bullet wounds are bloodless, and every death is an instantaneous drop. And, unlike cowboys, these martyrs are holy characters. It would be not only gruesome, it would be undignified to show them dying in hideous distress.

Or perhaps the point is that the pictures are realistic. If they look odd, it's because they show something strange: a miracle. Someone is being tortured, but through divine intervention they are supernaturally anaesthetised. God likes martyrs, and spares them pain, lets them die with serene dignity. And it's true that, in some legends, the saint is miraculously delivered and (Rasputin-like) comes back to life, only to be subjected to another worse death.

But none of these is the correct, or at least the official answer, to the screaming question. And Bennett's apparently jokey suggestion of a stiff upper lip is in fact nearest. Martyr means witness. Martyrdom is a public performance. Martyrs bear witness to their faith by undergoing torture and death without protest. These impassive figures actually show incredible endurance. It's like a torture scene in a war film, where the hero is phlegmatic to the bloody end. Though the martyr's obedient witness is not the same as the soldier's "grit", the spectacle is impressive in the same way.

There are some martyr pictures, though, where any explanation seems possible. Dieric Bouts' triptych of The Martyrdom of St Erasmus has one of the most horrible stories. Erasmus was an early Christian bishop, persecuted by the Emperor Diocletian, and finally killed by having his intestines wound out of him on a windlass. That's the episode the painting illustrates. But looking at it, what do you see? A miracle? Amazing fortitude? Squeamishness? Sheer ignorance?

The whole scene - the landscape, the line-up of figures - shows composure. There's no violence in the actions or expressions, no agony or anguish or cruelty. The two patron saints, St Jerome and St Bernard, stand in the wings and look on calmly. The three officials are uninterested, and the headman no more than interested. The executioners turn their spindle as if they had a roast on a spit, the older showing a little vigour, the younger a mild compunction, but without malice or grief.

The martyr himself is pinioned, but passive. He lies there, stripped to a cloth, his bishop's mitre set on the ground next to him, his face slightly hardened, his body relaxed and unresponding. Even his flesh is without any natural resistance. From a neat opening in his belly, a thin line of innards is extracted bloodlessly and vertically from the horizontal man, with no more strain than drawing a thread through a needle.

Maybe Bouts' idea is a clean, painless and symbolic re-enactment of the dreadful story, a hall of fame version. Still the effect is, in its way, deeply repulsive and frightening. Blood and guts is our standard idea of horror. But to have a body impassively disassembled, even while it's alive, is an android nightmare - a far greater disruption of human nature than being violently hacked up.

There is some evidence that martyrs could perform such feats of transhuman endurance. As their bodies were taken apart before their eyes, and before the watching crowd, they kept going. During the Elizabethan persecution of Catholics, a man was being disembowelled, and he just went on praying, till his executioner cried: "His heart is in my hand - and Christ is in his mouth!"