Bruegel the Elder, Pieter: The Strife of Lent with Shrovetide (1550s)

Caravaggio dismissed the work of a rival painter with the remark that his "canvases were playing cards". It's an occupational hazard for anyone painting images on a flat surface. However much the artist pursues a sense of deep space and rounded volume, there is the risk that the solid bodies he depicts will go as flat as those figures of kings and queens and knaves that appear on cards.

When modern painting reacted against the three-dimensional illusion, and began to pursue a "flat" look deliberately, the critics responded in much the same way. They compared the work of Manet or Cézanne to playing cards, or sign-painting, or popular prints - until, that is, they learnt to love flatness, and wouldn't stand for anything else. But there are pictures in which flatness is used tactically, to introduce a strange spatial effect. The figures in this picture represent solid bodies, but they seem to be flat as well - to have actually turned into flat shapes with sharp edges, lying upon one another. Their flatness is more than a look, whether an error or a style-choice. It is made to feel like a fact about them.

The effect is used movingly in Russian icon paintings. These images treat every figure as a thin, sharp shape. When such figures come into contact, it's as if they were overlapping and interleaving like pieces of paper. In icons of the Virgin and Child, the tender cheek-to-cheek contact of embracing mother and baby is a simple overlap of shapes. Their faces are such slivers, and lie pressed perfectly flat upon each other. In other words, the figures are given an impossible intimacy, a more-than-bodily closeness - for no one ever really stuck to anyone like a stamp to a letter.

The effect is used cleverly in illustrations to Alice in Wonderland. Several of the characters in this dream story are, of course, playing cards. But Lewis Carroll often describes them as if, at the same time, they are also bodies occupying extended space. This is perfectly OK as dream logic. In a purely verbal description, no problem need arise. But an illustrator must decide what things look like.

If you take John Tenniel's Alice illustrations, you find he has various solutions. The Queen of Hearts is sometimes drawn simply as a monstrously solid woman, nothing flat or card-like about her. But there are also pictures, showing the assembled court, where the figures are drawn in a very flat way, in stiff poses. Though these characters are bodies standing around in a garden, they also appear to be as flat as overlaid sheets of cut-out card. The depicted world is deep and flat at the same time. The dream logic is preserved.

The essential ambiguity can be put like this: imagine a round shape in a picture. It can be a ball or a plate. It can represent a sphere, or it can represent a flat disc that's seen face on: it depends on what it's meant to be, on the way it's shaded in, and on the way it relates spatially to other things in the scene. Sometimes you get contradictory signals, as in icons and in Alice, where bodies are both solids and flats. Or there can be a local outbreak of flatness. A form that's voluminous is suddenly treated as if it were thin. It's impossible, and can only exist in a picture.

The effect is used cruelly in this picture by Bruegel. It's a comic painting, obviously, but it deploys volume-ambiguity to disturbing effect. It shows three heads, and it's called The Strife of Lent with Shrovetide, or simply A Fat Man and Two Thin Ones. The basic contrast/conflict occurs in other Bruegel pictures. The annual crunch point comes at Carnival (carne vale, "goodbye meat"), the end of eating and the beginning of fasting. Shrove Tuesday, Pancake Day, is the final blow-out before the lean weeks of Lent. It fell this week.

The fat friar is round-faced, his chin a mere dimple in his curving jowls, a smug smile on his lips. He is himself a pudding, fattened up for the duration. And he is confronted by two wretched, starveling, characters. One is in jagged profile. The other, driven by hunger and envy, lurches forward to take a bite out of the friar's cheek. Eat the rich!

It's an impossible bite - or rather, the bite makes it an impossible face. The starveling's munch is only a small nip. And you wouldn't get your face round something with such a small nip unless the thing was itself pretty flat. The friar's face here is both fat and flat. Its near, unbitten side is solidly chubby flesh. But the bitten side, within that narrow bite, is given a shallow, rim-like shading around the cheek and jowl, to make it as flat as a pancake.

It's not only the act of someone eating another's face that gets to you. This sudden jump from solid to flat, within the same object, is a piece of violence itself.