Sassetta: The Meeting of St Anthony Abbot and St Paul the Hermit (c1440)

The modern strip cartoon has enormous representational resources. It's a form of image that can do all kinds of things that traditional paintings can't. For example, cartoon characters can speak and think, through speech-balloons and thought-bubbles. And using various graphic marks, cartoons can indicate noises, smells, heat, throbbing pain, the direction of someone's gaze etc.

Cartoons have timing devices. With shudder-lines and whoosh-lines, they can show a range of motions. With extended whoosh-trails, they can trace a moving figure's recent path through the world. By drawing figures in multiple positions, they can depict actions (wagging tails, heads shaking) that elude painting. And, of course, through their frame-sequences, cartoon strips can tell a story made up of many successive events.

Unlike paintings? Well, narrative has been a sore point for the art of painting. Strict neoclassical critics used to insist that a single picture cannot hold a multi-episode story. They explained that a picture is a still image - therefore, what it shows is momentary, a frozen instant. Or that a picture shows a single space - therefore, all action within this space is simultaneous.

These arguments are arguable. Paintings have disregarded them, and even the strictest critics allowed some time-stretch. They didn't mind when a picture showed, say, someone's action and someone else's reaction to it. What they minded was when the same figure appeared several times in the same scene, in a sequence of incidents. Absurd - as if you could walk across a field, and shake hands with your former self!

Yet the multi-episode scene is often used by paintings (and not only "primitive" pre-Renaissance paintings). The practice has acquired a technical name - "continuous narrative". Sometimes it's done with just a two-stage, near-far split. You get the main event in the foreground, and in the background there's a smaller incident, with the same character, but showing what happened "before" or "after".

But continuous narrative is best suited to a journey-story, where a character travels through space and time together. The picture shows a landscape, with the figure seen at different points along their journey. Strip cartoons sometimes do a similar thing. The strip shows a character walking along a road, past trees and houses. In each frame, they're a bit further along. But if you removed the frame divisions, you'd see a single joined-up view of a road with the same figure at three different points along it.

This is what Sassetta shows in The Meeting of St Anthony Abbot and St Paul the Hermit. It's a sub-section of an altarpiece, and its story is about cave-dwelling hermits in the Egyptian desert, in the early centuries of Christianity. The aged St Anthony, in his vanity, thinks he was the very first hermit. So God sends him on a trek to meet the even more senior St Paul. En route, he encounters two creatures, first a centaur, then a satyr - demonic figures, who nevertheless helpfully point him on the right road. He finds St Paul, and they miraculously recognise one another. This Sienese painter is an expert storyteller. He paces his journey-story finely. He leaves out one of the monster meetings, and turns the tale into a classic one-two-three. First, the saint enters at top left, small and alone. Second, on the other side, a bit lower down, a bit bigger, he meets the centaur. And third, much lower, right down at the bottom, and much bigger, and almost centre, he greets his fellow-hermit with a massive hug. One-two-three: their embrace comes with a lurch, a big "at last!". It is also a hard stop. The traveller comes up against an identical figure, clinching in stable symmetry. (Both old men have dropped their sticks on the road.)

The scene is a unified landscape, not a strip. But Sassetta divides up the story into something like separate frames. The three incidents are visually isolated from one another, by the dark mass of the trees and the ridge of rock. What's more, between his three appearances, we see that St Anthony will go out of view. First, he will pass behind the mountain. Then he'll pass through the thick wood. Each time, there's a barrier between him and his next encounter - a way of showing that he doesn't know what will happen next.

This is picked up by the winding path. Altogether it makes a reversed "S", but it, too, keeps disappearing and reappearing, and usually it reappears not quite where you'd expect. It goes behind things. It goes off-picture. The path enters from the edge, and each time it turns, it goes outside the picture's edge and then comes back in again. These exits and entrances introduce extra time, and extra surprise, into the story. The path's final steep slant makes a sudden, rapid home-run.

So the picture tells its story stage by stage. All the same, a picture-story is quite different from a strip-cartoon, or a written tale, or a movie. As the neoclassical critics pointed out, in a "continuous narrative" scene, successive events occur together in the same locale. For the viewer, unlike the travelling saint, there is no "what happens next?".

We see it all. The "before" and "after" are simultaneous. The picture binds them into a visual pattern. (Mountain echoes cave-mouth echoes embrace.) The journey's end is already waiting, as it begins. It's still beginning when it ends. It is seen as if by the eye of providence. Its trajectory is a visible shape. Its conclusions are foregone. Human life here is both an unfolding journey, with turns and surprises, and a comprehended whole.

THE ARTIST

Sassetta (c1392-1450), real name Stefano di Giovanni, was the leading painter of his time in Siena. He belonged to the second Sienese school, a "retro" movement. Though aware of the technical advances of Florentine painting, he looked back to the style of a century earlier, with its wonderful sense of place.

As Timothy Hyman puts it in his classic study Sienese Painting: "Above the gate, inside the walls, below the towered hill, across the chessboard expanse of landscape, towards the curving horizon - no other art has engaged so imaginatively with the experience of moving about in one's own city, of hovering above it, of surveying the panoramic l andscape."