della Francesca, Piero: The Resurrection (c1465)

And when at last one has arrived at Sansepolcro, what is there to be seen? A little town surrounded by walls, set in a broad valley between hills' some fine Renaissance palaces with pretty balconies of wrought iron' a not very interesting church' and, finally, the best picture in the world."

Sometimes an artwork gets a critical catchphrase fixed to it, and can't brush it off. The phrase hangs around it for ever, and any critic who subsequently talks about the work must either make a nod to the phrase, or deliberately "cut" it.

Ingres's Portrait of M Bertin in the Louvre is eternally labelled with Manet's beautiful quip, "the Buddha of the Bourgeoisie". Jackson Pollock's drip painting is "apocalyptic wallpaper". Waiting for Godot is the play where "nothing happens -twice". Or it may just be a single word. Mona Lisa: "smile". But Piero della Francesca's The Resurrection is simply "the best picture".

Aldous Huxley wrote an essay with that title in 1925. The phrase was making a bid for posterity. Such a high and defiantly confident judgement from such a prominent pen was asking to be noticed and remembered. It was all the more dramatic because, as Huxley notes several times, the picture wasn't well known.

Sansepolcro is now on the "Piero trail" taken by many cultural tourists: Arezzo - Monterchi - Borgo Sansepolcro - Urbino. In the 1920s, it was quite hard to get to. Piero himself, though benefiting from a recent revival, wasn't as appreciated as (say) Botticelli, and no doubt for the reason Huxley gives: the big Botticellis were in the Uffizi, and Florence was on the main railway. The Piero towns were approached up mountain roads.

So "the best picture" was not an image that would spring to everyone's mind's eye. Colour-plate coffee-table art-books of Piero weren't readily available. The little volume in which Huxley's essay originally appeared, Along the Road, was unil-lustrated. Many of its first readers would have had no inkling what this supreme masterpiece looked like. And even given the limited power of words to evoke unseen pictures, Huxley's own description of it is fairly skimpy.

His general account of Piero della Francesca is good, sensitive, standard. He says what any half-decent critic of the time would have said (or today, come to that): hard, smooth, rounded surfaces' geometrical forms and compositions' stiff and solid bodies' severe moral tone' contained passion' dignified, grand, classical, intellectual. The only word in the Piero phrase-book that Huxley omits is "mathematical", though there's lots of talk of triangles, rectangles, cylinders, cones, perpendiculars, concaves and convexes.

There's a good deal of evocative name-dropping too. Piero recalls Egyptian sculpture. He is majestic like Handel, but not like Wagner. His vision of man is Plutarch's, not Christianity's. He is to Botticelli as Alberti is to Brunelleschi. The battle scene in the Arezzo frescos is "as though Bach had written the 1812 Overture". In fact, you might expect to hear more about Bach. He's another Christian artist who wins over the secular with his grand humanity, his mathematical passion - and he was getting big in the 1920s too.

As critical comments go, most of those are fair enough. But once you take a long and detached view of taste and criticism, it begins to look a bit Pavlovian. Huxley's high praise for Piero - it's another manifestation of the modernist/neoclassical tendency, with its delight in the impersonal, its attraction to the severe, rigid, ritual aspects of art. In 1925 Picas - so had just emerged from a serious flirtation with Poussin. The following year, Stravinsky wrote his hieratic, statuesque oratorio, Oedipus Rex. Huxley was punctually of his time.

In another way, though, the essay harks us back to an enviable moment. It was a time when you could be a renaissance art lover, and not yet know this picture. Of course, that gives Huxley's pronouncement some snob cachet, which is slightly annoying (as he's aware). But it also means that, inspired by it, people would now be trekking out to Sansepolcro to see Piero's Resurrection, see it for the first time.

"The best picture in the world is painted in fresco on the wall of a room in the town hall." It's still there, and still surprising in the directness of its presence, just painted on the wall at the end of a plain room, with no build up, waiting to meet you - waiting for you to meet the force of its full-frontal encounter: the resurrecting figure, that's standing up, and rising upwards, and heading towards you.

There is the upright head, with its absolutely frontal, eye-balling, madly symmetrical face. And straight beneath it, there is the upright torso, which pointedly straightens up and fills out above the curve-in of the left thigh. There's the high-raised right knee, foot planted flat on the tomb's edge, an extreme pose which suggests that, with miraculous power, upon this knee as on a hydraulic piston the body will lever and levitate itself up into the air.

It is a figure whose ascent, upright stance and face-to-face confrontation make a single impact' a figure full of summoning power, but powered by forces not all its own, mesmerising and mesmerised. If you want a best picture, you could do worse.