Walking round Tate Modern's Rousseau exhibition, I found myself engaged in a slightly odd mental exercise. "What would these paintings look like," I wondered, "if he was a better painter?" The answer in most cases was "absolutely awful". Just think of these works replicated in the slick pompier style to which Rousseau aspired and all their charm decays in an instant.
Take Liberty Inviting Artists to Take Part in the 22nd exhibition of the Société des Artistes Indépendants, in which a neo-classical figure hovers above the entrance to the Paris exhibition hall. Frankly, it's risible as it is, with its well-drilled chorus line of exhibitors and awkwardly lofted goddess. But at least you laugh affectionately.
Imagine it in the full, florid solemnity of neo-classical academic style, however, and it would become irredeemably ludicrous. Even the classical jungle scenes - often marvels of colour and composition - wouldn't survive the procedure. What operates now as a kind of mysterious dream imagery would be reduced to mere set-dressing, a bit of exotic bric-a-brac in line with the fashions of the time. So, as you walk from picture to picture, you can't help thinking of Rousseau as an artist shepherded towards greatness by his own lack of skill as a painter.
This needs elaboration, because "skill" is a tendentious term. Like many, I first encountered Rousseau's great painting Tiger in a Tropical Storm when I was just starting to look at paintings (naive art usually goes down well with naive audiences). I liked it so much that I bought a big reproduction for my bedroom, which - in a fit of misplaced aestheticism - I'd painted chestnut brown, ceiling and all. Curiously, against that background it looked rather well, its colours even richer. But I couldn't really remember what I'd thought was good about it - and entering the Rousseau exhibition, I feared that my teenage enthusiasm might turn out to be an adolescent crush. Would it go the way of my passion for the Strawbs' Grave New World and the novels of Russell Braddon?
Well, not at all, as it happened. Tiger in a Tropical Storm - one of the first paintings you see in the exhibition - is still a showstopper. This isn't because it possesses a sophistication or undertow that I'd missed on first inspection. It is so devoid of menace that it would be perfectly at home as a frieze on the wall of a day-nursery. But it achieves through lack of sophistication something that feels undeniably skilful. I suspect it's quite a hind-brain affair - something to do with primal responses to pattern and rhythm - but it certainly isn't about subtlety. Rousseau's solution to the venerable technical problem of how you paint rain is simply to overpaint it as thick translucent stripes. And yet the dapple of light that results is very beautiful.
The crudity of the technique is the point, of course. The simplicity of the means was what Apollinaire and Picasso liked about Rousseau; and even if you suspect they valued the douanier as a stick with which to beat their enemies, rather than as any kind of intellectual equal, they understood that he had a quality that academy paintings could not achieve.
Ever since then naivety - real and artfully simulated - has been part of the repertoire of high art. What's more, it may only be visual art that can properly deliver this odd combination of cluelessness and achievement. Could you have a naive play or novel, in the way Tiger in a Tropical Storm is naive? You wouldn't have any difficulty in turning up untutored talents: after all, playwrights don't generally attend playwriting school or apprentice themselves to a master. But the very process of publication and performance would tend to corrupt the innocence you treasure.
Rousseau's career is instructive, in this regard. He also wrote plays - which I assume are as guilelessly ingenuous - but they never received a production, and there really wasn't much Rousseau could do about that. His art, on the other hand, was effectively insulated from the consequences of public and professional disregard. A painting needs no collaborative rehearsal or editing process. So, year after year, Rousseau doggedly included his work in the Salon des Indépendants until people began to look forward to seeing them - first as a joke, and then as a joke that had mysteriously turned serious. What he didn't do was improve; some of the most cack-handed paintings here come from relatively late in his career. It's a blessing, really, for him as well as us. If he'd been any better as a painter we probably wouldn't have heard of him at all.Reuse content