Paintings visualise the world for us. They observe or imagine what it looks like. They show us the sight of things. But not always. Some paintings do just the opposite. They show us something, while declaring that they've no idea how it really was.

Take the early pictures of René Magritte. Ask, what do they know? The answer, usually, is not much. They know three or four facts. They know that certain things were the case – unaccountable things, often, even quite incredible things. But as to how these things managed to be the case, the pictures do nothing to help us see.

The real mystery of these scenes doesn't lie in the odd facts they depict. It lies in the way that a shadow falls across this depiction. The picture itself hasn't observed these things. It has somehow heard about them, read about them, remembered them, learnt them in a dream – and then translated them into a visual image in the most literal and unimaginative manner.

Most illustrations try to visually realise their subjects. But Magritte's pictures don't add realistic details or personal touches to the basic information that (it seems) they have in their possession. There's no attempt to fill in gaps or make the sight look plausible.

Many points are left blank or general. Where they must choose a particular kind of thing, like a key or a suitcase, they choose the most standard instance, the textbook or ABC example, the specimen that tries to say key or suitcase and nothing more.

So the scene, strange in itself, is estranged further by being set at a remove of knowledge. It is painted so as to suggest that the artist didn't know, couldn't conceive, what it looked like. He was granted certain bare facts, in a void. His image stays true to them alone. Whatever more happened, what a witness might have seen, is quite beyond his or our grasp.

Attempt at the Impossible is a painting about painting. It's an artist-in-his-studio picture, showing a painter and the image that he's at work on. But of course there's a big strangeness. If you ask, what the picture knows, you could say it's this: there was a painter, painting the figure of a woman, but painting her on to thin air. This woman was life-size, upright, naked, and no less real than him.

Eh? But that's all we see, just those extraordinary facts, illustrated in the most literal way. The floorboards and the dado, the artist's suit and haircut, the model's hairdo, all might come from a picture book. There's no sense that the image is grounded in experience, or that Magritte has tried to visually reconstruct the scene. If you wonder, how so? – how could an image be painted on to air, or be as substantial as an actual body? – the picture makes no claim to understand.

It has only this minimal information. It asserts bluntly that this is the way things were. The woman's body casts shadows just like his, while her flesh stops sharply at an edge where his brush stops. Incredible! In other hands, a painting would have tried to make this incredible proposition somehow visually credible. Here you feel only a fog of unknowability.

Low-level visualisation is early Magritte's trick. (Later his painting became more expert.) The woman's body emerging under the painter's brush is painted to the same standard as his own – and it's such a perfunctory standard of realisation that it raises no expectations that it can elucidate anything.

What this scene is meant to be of, or about, is in many ways open. The woman can switch before our eyes between being a painted image, a living model, a solid statue, an inflatable doll, a thought bubble. Is she a two-dimensional, three-dimensional or hallucinatory figure? Is this a conjuring trick or an illusion in the painter's own mind? Is it a kind of miracle or merely a metaphor?

Attempt at the Impossible can preach almost any moral you like about painting and sex and imagination. The motive of painting is to embody sexual fantasies. To paint a figure is to touch or possess someone by proxy. Painting is about immobilising life. Sex is about immobilising life. Desire is always really directed towards a figment of your imagination. Imagination's object can never be grasped in reality. Our imaginings are as real as we are.

All these interpretations or others will do. A few plain inexplicable facts are what the painting provides. Once there may have been a complete picture, a full story, of what was going on. But now the artist is helpless to reveal more. With a dark, heavy, sluggish hand, he puts together the fragments of a vision that, if it was ever his, has entirely passed from him.

The artist

René Magritte (1898-1967) is of course a paradox. A Belgian surrealist, popular and avant-garde, he's the straight man who painted bizarre scenes in a deadpan manner. But he makes sense. He couldn't paint very well, but his work is sustained exploration of the language of images. With a vocabulary of brick walls, clouds, apples, nudes, eyes, rocks, bowler-hats and handwriting, he's always making a point about how pictures work and how strange they are. He plays with scale, perspective, shadow, illusion. He revels in metamorphosis, in the weightlessness of the pictured world, the way you can never know what's behind something. In Magritte, a picture becomes a place where everything is trapped and anything is possible.