Cotan, Juan Sanchez: Still Life with Quince, Cabbage, Melon and Cucumber (c1600)

Images are mostly not for looking at. They are for being there and having around. Take family photos. A grandmother, say, sits in her sitting room, surrounded by framed pictures that are set up on shelves and mantelpiece. They show the various members of her family, living and dead and newly born. They show children at different stages of their growing up. They show family occasions.

And at the next performance, Bankhead goes to work. Making her exit at the end of their scene together, she pointedly sets down a glass of champagne (with some adhesive on its base) over the very edge of a table. There it remains for the rest of the act, impossibly balanced, stealing all eyes.

But actual impossibility may not be essential. Accidents can always happen, and we have a keen eye for potential disaster. As Pascal said: "If you put the world's greatest philosopher on a plank wider than he needs, but with a precipice beneath, however strongly his reason may convince him of his safety, his imagination will prevail."

Precariousness: something so nearly is, so easily could be, otherwise, you can't help imagining a turnaround. It happens in pictures too. In fact, the Bankhead stage trick might have been taken from painting. There's a favourite device of still-life artists who want to put suspense into the inanimate. A plate, a basket, a knife, is projected out over the edge of a table, so that its centre of gravity liesjust short of that edge. A jog might send it over. In the stillness of the scene, there's a fall impending.

That's simply done. Things in pictures are not under gravity. But just a hint of jeopardy will usually do. Many still-life pictures employ this effect, and generally they want only a faint frisson of instability. They don't want to upstage the whole scene with a single object that lies in fascinatingly miraculous equilibrium. Then again, there are some still lifes where almost everything - one way or another - is on tenterhooks.

Juan Sanchez Cotan's Still Life with Quince, Cabbage, Melon and Cucumber is one of the pinnacles of the art. In this scene, fruit and veg are translated into pure form. Within an oblong stone frame, five ingredients are displayed: a quince, a cabbage, a cut melon, a slice of melon, a cucumber. Each object is picked out separately against the darkness or the smooth stone. Each is modelled firmly in the cool, sharp raking light. They are like geometrical solids. Each one is exactly placed.

They are arranged in a regular descending curve - a hyperbola, to be precise. This is achieved partly through the scale of the things, and partly by suspending two of them up on strings. Down they come, one by one, step by shallowing step. And at the end of the curve there's another bit of geometric punctuation. The diagonal shadow of the cucumber cuts off the surface of the sill so as to leave a neat isosceles triangle.

It is a supremely graceful layout, but more than that, it is a dramatic one. The drama is in the tension between the curve and the objects that compose it. The curve holds its objects, imposes an order on them. On the other hand, the objects are in principle moveable. They could well lose their exact positions in the sequence. They put the perfect curve under a pressure of precariousness.

All five objects are still, but only one is absolutely stable. The central dissected melon sits safely grounded on the stone sill, and safely back from the brink. With the other four, their stability and stasis are at risk.

You notice how the curve does not simply descend within the window frame. It goes slightly aslant to the frame's edge. It curves out, towards the front of the picture, across the lip of the sill. It brings the melon wedge and the cucumber forward, and they perform the standard still-life suspense trick. They project out over the edge.

They are probably not too far gone. But they look as if they might - with only a little nudge, a little shove - tip over. How easily the melon wedge might rock on its hard, curved skin. How easily the cucumber might cantilever. There's only a margin of sill between where they sit and a sheer drop. The shadows they cast beneath them foreshadow their potential fall.

Now go to the top end of the curve, to the two dangling objects. Their positions are even more dicey. Look especially at the marvellous cabbage, hanging heavily in mid-air on its long, fine string. It is certainly in a precarious state. It's not that it might drop. There's no reason to think the string will break. The thing that might be broken is the cabbage's stillness.

We understand it to be perfectly motionless, but how vulnerable that stillness must be. It is about the same size as the sitting melon just below it, but its physical situation is conspicuously different. It hangs there, utterly exposed and unsupported. And as with any highly fragile but stationary situation, its stillness acquires a kind of aura. It's like a house of cards. The slightest touch or draught, the least unwinding in the string's fibres, would make the cabbage sway or turn, and lose its place. It is the weakest link in the hyperbola.

So there is a play to and fro between the unnaturally perfect curve and the naturally precarious objects that make it up. You can look at the scene and see an exquisite, almost transcendent order that embraces the world. Or you can see a display of contingency, a very deliberate and fragile construction. One firmly settled object, the central melon, is flanked by things whose positions are both contrived and perilous. At any moment they could slip, tremble, swing, succumb to gravity or change, and destroy the pattern entirely.

Both Tallulah Bankhead and the greatest philosopher might be impressed.

THE ARTIST

Juan Sanchez Cotan (1560-1627) was one of the earliest pure still lifers in European art, and one of the most piercing. The fame of this Spanish painter rests on a small number of pictures, all using the same format, a stone window frame, within which highly realistic items of food sit or dangle - game-birds, tiny apples, the mighty cardoon. It is a minimal art of few ingredients, in which form and colour can be tightly controlled. In mid-life, Cotan chose a religious vocation, becoming a lay-brother in the Carthusian order, and it is tempting to see his still lifes as sacramental, or at least frugal. His explicitly religious paintings have not proved nearly so popular.