Thomas Sutcliffe: Michelangelo is not as perfect as he's drawn

The compositional analysis of genius tends to roughly divide into two schools: supernatural inspiration or human effort. One set of recipes emphasises the inexplicable nature of great talent and treats it as a kind of divine fluid poured into a receptive vessel. Another set insists on the sheer labour required to transform abstract genius into real effects in the world. Edison's remark about genius being 1 per cent inspiration and 99 per cent perspiration is probably the most famous of these - but Michelangelo offers a version of it with his line: "Genius is eternal patience."

Patience with what, though? The question comes to mind as you walk round the British Museum's new exhibition, Michelangelo Drawings: Closer to the Master - a show, in effect, devoted to what fills the gap between an idea and a finished masterpiece. Michelangelo was famously cagey about his drawings, and indeed made a bonfire of many of them before his death. But here, those private workings are exposed to the closest scrutiny. And it's part of the magic of the show that you can lean in close to see each individual line and smudge of chalk.

For some, close inspection has been problematic. The critic Richard Dorment, writing in The Daily Telegraph, criticised the exhibition for not directly addressing the tricky question of Michelangelo attributions - and he used the evidence of his eyes to back up his suggestion that many drawings here may not be by Michelangelo at all. In doing so, he raised a real issue. How, when even a signature isn't indisputable evidence, can we know who really did these drawings? The official roster of uncontested Michelangelo drawings has fluctuated constantly, buffeted this way and that by curatorial pride and rival scholarship. And, without documentary evidence, it's understandable that people turn to connoisseurship - to deciding whether something looks like a Michelangelo or not.

Dorment's notion of the Michelangelesque, it turns out, is very exacting. Fortunately, it's also unmissable. "When you stand in front of a drawing by him its quality leaps out at you," he writes. Part of this quality is infallibility: "Michelangelo never makes a mistake in the proportions of a body, or misjudges the thickness of a line." Never? Yes. He goes further: "From the age of 12 Michelangelo drew perfectly." On the basis of this, he's pretty sniffy about several of the exhibits in the show. If they aren't perfect, the thinking seems to go, then they can't be Michelangelo.

These seem to me to be demeaning things to say about a great artist, rather than reverential ones - and they provoke the obvious question, which is how on earth Dorment knows. At which point, the catch-22 circularity of attribution on aesthetic grounds should become obvious. What's the evidence that Michelangelo drew perfectly from the age of 12? Well, the fact that all the uncontested drawings are perfect. And why are they uncontested? Because they appear perfect and thus fit the fantasy of the artist whose hand never wavers. Once you have decided that the hallmark of genius is perfection, you can comfortably exclude any work that disables that assumption.

Like Dorment, I don't think every drawing in this exhibition is a masterpiece. There's a drawing of the Risen Christ (Catalogue No 76) which looked very dodgy to me - strangely lumpy in its musculature and oddly pin-headed as well, for an artist with such fabled command of proportion. But, unlike Dorment, I can think of a lot of explanations for its look short of the fact that it was drawn by someone else entirely. And the psychology of Michelangelo's reticence about his drawings has to be taken into account. That shyness surely has its origins in the knowledge that there's a difference between preparatory sketches and finished work. And that difference has to involve a sense of imperfection. Perhaps Michelangelo was doing a bit of mythologising himself here - actively preserving the notion of unerring, god-like touch. But if he truly was unerring, where does patience come into the equation?

It seems more likely - and more humanly true - that for Michelangelo, drawing wasn't simply a way of thinking on paper but also a safe and private place to fall short of his reputation. That's why he guarded it so fiercely. When drawing he could try out alternatives, test a composition, rehearse the effect of a line or the interplay of shadow and light. And what his genius had to be patient with were the occasions on which he concluded that this wouldn't quite do yet. The less than perfect pictures in the British Museum show don't detract from the unquestioned masterpieces it also contains - they provide a proper frame for them.