Philipp Otto, Runge: The Child in the Meadow, from Morning (1809)

A work of art, or so they say, should be more than the sum of its parts. Maybe. But it is also the sum of its parts. And it is also its parts, one by one. Admittedly, when it comes to pictures, it may not be so obvious what the parts are. Unlike a text, a picture has no clear internal breaks. It doesn't divide up into words and sentences, lines and paragraphs. When you take a detail from a painting, its exact size and shape and edges are up to you. Still, pictures do often visibly come to pieces.

Just as literature has its famous quotations, painting has its famous details. Look at the postcard racks in any gallery shop if you want to know what they are (an obvious example would be the almost-touching hands of God and Adam from Michelangelo's fresco). But perhaps this approach seems somehow wrong. We're taught that an art work should be a unified, harmonious whole. So it may feel frivolous or philistine to have an eye just for the good bits.

Indeed, it may cause us to overlook the fact that many artists of the past weren't only trying to be harmonious, etc. Just like writers, they were trying to be quotable, too. And when you read traditional art criticism, you find that the critics are always pointing out this or that detail as especially striking and worthy of admiration. From the ancient-Greek painter Timanthes, remembered for a single touch " how he portrayed the grief-stricken face of Agamemnon hidden behind a cloak " the history of art has often been a parade of telling details. But there are pictures in which a detail doesn't merely catch the eye or steal the scene. It demands to be a picture all of its own.

The main picture here is Morning from Philipp Otto Runge's great work, Times of Day. This was a painting sequence that consisted of four allegorical scenes " Morning, Day, Evening, Night " depicting a universal, elemental mythology. The images, tall as altar paintings, showed earth and sky, figures, flowers and stars in geometrical compositions. Their forms and colour schemes were carefully calculated. They were to be displayed in a purpose-built sanctuary, accompanied by music and poetry.

Ars longa, vita brevis: at his early death, Runge still hadn't completed the first picture, Morning. He suggested that the unfinished canvas should be cut up into its more or less finished parts. Later, it was. One of these pieces shows a naked baby lying on the ground. It comes from the middle of the bottom of Morning. As a fragment, it was given a new title of its own, The Child in the Meadow.

And it works very well as a self-sufficient image. Although the separated parts of Morning have now been stuck back together again, with missing blank areas, Runge's deathbed solution did make sense. Morning as a whole was, and is, clearly composed of isolated incidents. It could easily be cut up into pieces, without cutting anything important in half. What is more, this particular fragment, partly because it is a fragment, acquires a peculiar power.

The baby: it's not Jesus, not precisely. It looks like Jesus, of course, very like him, as he appears in many Nativity scenes, lying naked on the ground, with quite a bit of space around him. The echo is plain, and plainly intended. So is the difference. This is a divine baby, yes " but non-specifically divine. It's not the Virgin-born Christ-child of the Christians. It's a universal symbol of the miracle of birth. (And, unlike in many Nativities, Runge leaves the child's sex unclear.)

In other words, the picture performs a kind of collage. The Child in the Meadow, this detail cut out from Morning, is itself like a detail cut out from a typical Nativity painting. It extracts the baby alone, removes it from its specifically Christian context and story " Mary, Joseph, shepherds, angels " but holds on to its aura of divinity, its miraculousness.

Now, in the full picture of Morning, Runge gives the extracted baby a new context. He sets it in an elaborate, rather abstract mythological scene of his own devising, with a skyful of naked symbolic figures overhead. But when you see the fragment, it's clear the new context isn't needed either. You need no more than the detail alone: a divine baby, taken out of Christian theology, and laid in nature.

All by itself, the little scene generates an elemental drama: a drama between earth below and sky above, and human creature in between. Lying flat on its back on the world's flat surface, the baby is emphatically grounded, under gravity. Facing and gazing straight upwards, it bathes in the morning light that falls from the sky " the sky off-picture, but strongly implied by the glow reflected on the baby's body. It opens its arms in welcome.

The new arrival is like a creature landed, fallen from the sky, an extraterrestrial materialising on earth. Equally, it's like a flower growing out of the fertile ground and opening to the sun. And because the fragment is tightly cropped, the whole picture belongs to the baby. It lies there, isolated from any wider human context, in a world of its own, a small thing but wholly self-sufficient.

The cut-out detail becomes an extraordinary image of new life, of pure beginning. The child's origins are made far more miraculous than those of the Virgin-born Jesus " or of any normal baby, of course, with its mother and father, its genetic history and its social setting. This fragment is a radical nativity scene that corresponds to our sense that every child (even if we have made it ourself) is also a strangely new thing, entirely original, individual, arriving out of nowhere.

THE ARTIST

Philipp Otto Runge (1777-1810) is best known for his portraits of children, terrifying little giants, bursting with chunky vitality. He was the most original of the German Romantic painters. In his short life, he initiated a number of artistic projects that took off in the later 19th and 20th centuries. He explored the growing forms of plants (in semi-abstract silhouettes), for example, and the physics and symbolism of colours. In Times of Day, he envisaged the 'total work of art', a visual-verbal-musical-installation experience, a dream that has inspired, in very different ways, artists from Richard Wagner to Mark Rothko, and still hasn't quite lost its shine today.