de La Tour, Georges: The Dream of St Joseph (c1640)

We know you saw something, says the scientific investigator to the haunted heroine in the classic scary film. We know you had some kind of experience. We don't think you're making it up. But as to whether what you saw was a genuine visitation, or all in the mind, or a trick of the light, we're not going to jump to conclusions. After all, we are living in the 20th century.

But, as every cinema-goer knows, such cautious scepticism is always unfounded. The hard-headed rationalist is just blind to the obvious. The credulous hysteric is the trustworthy witness. And the wildest possible explanation will turn out to be the true one. It wasn't a delusion, a misperception. It really was a ghost, a demon, a monster - and any minute now it'll be back with all its friends.

There are films, of course, and books that refrain from this straightforward trajectory. They genuinely don't know. They raise the idea of the spooky or the holy, and then suspend both belief and disbelief. They accept "unexplained" phenomena as truly unexplained, without resorting to natural or supernatural answers. There are pictures, also.

On three occasions in St Matthew's Gospel, an angel "suddenly appears in a dream" to Joseph. The first time, the angel tells him to marry Mary' the second, to take Mary and the newborn child off to Egypt' the third, to return home. And any of these dream appearances could be the subject of Georges de La Tour's painting called The Dream of St Joseph.

Or at least, that's one guess at this picture's subject. Other stories of angelic intervention have also been proposed - and modern-art scholarship, like modern cinema, prefers a supernatural explanation where possible. Yet in old catalogues, the picture has a more neutral title: An Old Man Asleep, Woken by a Girl Carrying a Candle. That description finds nothing miraculous here, just an everyday nocturnal incident. There is room for doubt, in other words. We see something - but what do we see? The image requires some psychical research.

Stick to the facts. De La Tour shows, in medium close-up, a child and an old man by candlelight. The candle is set between them on a small table. The old man sits, head in hand, an open book on his lap. He has nodded off. The child, probably a girl, stands in front of the sleeper, and makes a gesture of invocation. The old man's eyes are closed. The child gazes at him, but he does not look back at the child. He may be dreaming.

The child has no halo or wings (angels are, traditionally, male). And there is no sharp disjunction between man and child. They look as real and solid as one another. They're in the same space, the same light. They both have their feet on the ground. There's no suggestion that they belong to different zones of reality, or different levels of existence. There's nothing to mark the child out as a supernatural visitor, or as a dream figment (the old man's thought-bubble). The visible facts of the matter allow a wholly naturalistic account.

Up to a point. But these plain facts, as so often, leave out almost everything. They fail to convey what is solemn, visionary and numinous in this scene. The child's gesture hardly passes for everyday. It is a conjuring, spellbinding gesture, one of weaving or perhaps harp-playing, by which the child seems to be putting the man to sleep, or communicating with him in his sleep, sending him dreams. What's more, it isn't really true that the child and the man inhabit the same space and light. The spatial relationship between them is weird. At first glance, the child seems to stand nearer than the man. But check the man's knees: he now comes nearer. The child's right hand hangs uncertainly. Is it held out in mid-air? Is it almost touching the sleeper's wrist? The figures seem to occupy not quite congruent spaces.

Above all, there is the light. This is where the scene works its most ambiguous magic. It creates both the highest realism and the deepest mystery. The candle casts a soft, even, flickering glow over the old man's body, bringing out its solid forms. It turns the child into a kind of apparition. Its body is pure dark silhouette and sudden bright fragments - the raised hand, the radiant face. (The flame is carefully hidden from our sight, so that the light looks not like light from a source, but light emanating.) The child becomes a dream-vision or a visitation, half-dematerialised, gleaming out of the night into the dreamer's consciousness.

Yet, at every point, the painting keeps stressing its amazing powers of observation. See the curling backlit page, the translucent fingertips, the licking peak of the flame. As the mystery is conjured up, its mechanism is also made clear: tricks of the light.

What's remarkable about this picture is that, whether its subject is meant to be religious or secular, it stays completely equivocal about the supernatural. It shows an everyday incident and fills it with an overwhelming sense of holiness. Or it shows a miraculous incident, but includes nothing that can't be given a natural explanation. Even that strange gesture might be child's play - a child playing at being an angel? It can go either way.

Maybe this equivocation is a perfectly Christian attitude. The divine comes down to earth, becomes homely' the humblest human action is exalted, sacred' it doesn't matter if it's an angel or a child. Or maybe De La Tour's visionary picture points towards scepticism, a psychological understanding of religious experience. We have intimations of the beyond, the sacred, the spiritual, but whether these glimmerings reveal transcendent reality, or only the mind's own susceptibilities, who knows?

THE ARTIST

Georges de La Tour (1593-1652) is a shadowy figure. Not much is known of his life. He worked in Lorraine, and, somehow or other, picked up the influence of Caravaggio. He painted first daylight scenes of mainly lowlife characters, and then dramatically candlelit scenes of mainly religious subjects. His observational techniques are miraculous. His scene-setting is brilliantly calculated. But he keeps his distance from his human models.

Like Caravaggio, like Vermeer, De La Tour fell into obscurity until the late 19th century. There is no certain likeness of his face. His signature, however, is the most distinctively calligraphic of all the old masters.