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Great Works: Landscape with the Ashes of Phocion (1648), Nicolas Poussin

Walker Art Gallery, Liverpool

The city plays many roles. It is hell. It is a fairground. It is a madhouse. It is a celestial palace. It is a great machine. That's the role it's given in Walter Ruttmann's 1927 silent film, Berlin: Symphony of a Great City. Though the metaphor is supposedly musical, the visual effect is more mechanical. A day in the life of the city is translated, not into a symphonic structure, but into a series of routines and reiterations.

Doors opening; vehicles moving; crowds passing; humans walking, working, eating, dancing; wheels turning; gadgets shuttling: all these behaviours, living or not, are repetitive, and in their repetition they all become equal and one, a single driving incessant operation. Even when individuals briefly emerge, the effect is only to stress the pattern. Through sheer accumulation, automatism becomes is a kind of grandeur. We're enlarged by the whole into which we're all incorporated.

Or take another city vision, this time a still image. Nicolas Poussin's Landscape with the Ashes of Phocion, though it's called a landscape, is obviously a townscape. It's an imaginary view, created by an artist living in 17th-century Rome, of an ancient Greek city-state. This city is Megara, and the ashes... but the story can wait. Most of the painting's "story" is in Poussin's construction of a civilisation. And Poussin is famous for his order.

A picture makes order with spatial devices: symmetry, centring, verticals, horizontals, parallels. Poussin's city uses them vigorously. A row of massive oaks, heavy with foliage, runs straight across the foreground. It's a high natural city wall planted alongside the actual low wall, a great shady barricade. But it has an opening. These guardian trees frame the city like a pair of curtains or wings. A road runs between them, coming from outside. Bracketed by the dark trees, the luminous city appears beyond, a composition of arches, pillars, squares, rectangles, diagonals – buildings that suggest a set of toy bricks.

But it's clear at once that Poussin's order is unlike Ruttman's. Berlin has a uniform repetitiveness. It's a process with no climax. Megara has a structure. It's ruled by a centre and a hierarchy. The façade of the classical temple looks straight out at us, pretty well from the middle. It is the face of the city, the focal point of the whole scene. But this temple isn't the culmination. Standing behind it, higher, there's a great wooded rock, its two jagged molar stumps emerging from the vegetation covering its base.

The city was founded here, presumably, because of this wild and sacred natural formation. And the rock is not simply the city's heart. It is the peak of a pyramidal form that rises up gradually on each side from the ground, and which shelters the town, and seems to touch the sky. Above it there's a third element, a solid and graceful crown of cloud. Temple, rock, cloud: they stand like a mysterious symbol, the vertical alignment that sustains the civilisation.

Having noticed this organisation, though, notice also that it's complicated. It's wrong to overstress it. The scene has clear regularities, so it is tempting – for the sake of drama – to call it strictly regimented. You can say: the temple is dead centre, the city rises as a symmetrical pyramid, the wings of trees make a symmetrical frame, the temple-rock-cloud stands like a perfect vertical column.

But not quite. While the picture is on the verge of these geometries, it carefully avoids them. Its potential centring, symmetry, verticals, are shifted a step to the left. The design of Poussin's Megara may be more hierarchic that Ruttmann's Berlin. It's much less rigid too.

Within this structure, the life of the city freely flourishes. If you let your eye move among the specks of humanity that inhabit the flat ground and the heights, you pick up constant activity: walking, talking, reading, music, shooting at a target, swimming... and there's always another, smaller speck to notice. It's meant to be a little hard to take in. That is the measure of its freedom.

It's not a world where every inhabitant is being arranged into a visible pattern. Its life is going on regardless, going on without you. You, as you look at this scene, are an outsider, remote from the bright city and its happy pursuits, beyond the wall, in the deep shadow of the oaks.

In other words, it's a picture about exile. And this is where the story comes in. Phocion, an Athenian general, was falsely condemned and executed, and his unburied corpse banished, and taken to the outskirts of Megara, where it was burnt. At the very front, his faithful widow gathers his ashes. Her servant keeps lookout. The outcasts are directly below the mighty nucleus of temple-rock-cloud.

But nothing in the scene indicates that the civilisation from which they're excluded is itself evil or oppressive, that they're well out of it. No, their exile from the good life is sheer tragedy. This city is a symphony, and it continues behind them undiminished.

About the artist

Nicolas Poussin (1594-1665) is known as the most cerebral of painters. When the sculptor Bernini looked at his work, he remarked: "Mr Poussin is a painter who works from up here", tapping his forehead. Poussin, born in France, operated mostly from Rome. His painted "high" subjects – from the Bible, classical legend, ancient history, epic poetry. He constructed his scenes using model theatres, paying great attention to their visual and symbolic plotting. These were images for private study, for an intellectual elite, not public works for church or state. But their patient deliberation can accumulate into to a massive force, and they're capable of articulating the most violent emotions. A mouth in a Poussin painting was a crucial inspiration to Francis Bacon's screams. His work was much collected in Britain, and is well represented in the National Galleries of London and Edinburgh.