You know the type. He's the guy at your high school reunion who just quit his job (dull to start with) and cut loose from his family (wife deceased, kids farmed out to relatives), in order to devote himself entirely to his art. And when you see that gleam in his eye, you don't need to ask what kind of art. This type never wants to be a conceptual artist, exhibiting piles of toenail clippings or streaking through the financial district on a skateboard. Nor an installation artist, re-creating his own grungy bathroom in an even grungier downtown gallery. And definitely not a transgressive artist, running dead rats up flagpoles or nailing plastic Nazis to a cross. The Great Artist wannabe is typically just that - a Sunday painter with no real training, who earnestly believes that hard work and exalted thoughts will turn him into Titian ... or at least, William Bouguereau.

Did Henri Rousseau fit this bill? In a way, yes. It was definitely a clueless amateur who painted the dreary small landscapes of suburban Paris now filling one room of the Rousseau exhibition at the National Gallery. If you favor the postmodernist erasure of the line between high and low art, then you'll enjoy seeing daubs such as The Environs of Paris (1909), Banks of the Oise (1905), and Ivry Quay (1907) given the same royal treatment as the Venetian masters on display in the West Building. If you'd prefer not to see that line erased, then your reaction will be closer to that of the Paris Salon-goers who, accustomed to the lofty subject matter and polished technique of the Academy of Fine Arts, scoffed at Rousseau's doltishness.

At the same time, the early modernists - many of whom had been trained in the academic style, even as they spurned it - praised Rousseau's art as "naive," "primitive" even "folk," meaning basically that it was ignorant of anatomy and perspective. Picasso, who had mastered classical draftsmanship under the tutelage of his father almost before he could walk, paid five francs for Rousseau's gawky Portrait of a Woman, and called it "one of the most truthful French psychological portraits." (It is probably worth remembering that five francs was chicken feed - and that Picasso was a Spaniard.) In 1908, two years before Rousseau's death, Picasso and the poet Guillaume Apollinaire threw a banquet in his honor, attended by a who's who of the Parisian avant-garde. Accounts of that legendary event differ, but the overall tone seems to have been part mockery, part affection for the guest of honor, who, twenty years older than anyone else present, had never aspired to be part of the avant-garde.

This exhibition does a nice job of highlighting the mismatch between the modernists' embrace of Rousseau and his own somewhat deluded self-image. Modernism back then was not the insulting joke that postmodernism is today. On the contrary, Rousseau's fan club included some of the most significant artists of the twentieth century. But Rousseau had little use for their work. His hero was Bouguereau, the ultimate academic painter, and he bragged of having received advice from two others, Jean Gérôme and Félix-Auguste Clément. But no advice, however kindly or condescendingly given, could substitute for the rigorous course of study enforced at the Academy. (For example, students were forbidden to touch a paintbrush until they had completed several years of figure drawing and accumulated an acceptable portfolio.)

Rousseau's self-delusion is perhaps best revealed in John House's contribution to the catalogue, which recalls him "submitting designs to the competitions for the decoration of several of the town halls in the Paris region - those of Bagnolet, Vincennes and Asnières." As House adds, "town-hall decorations were perhaps the single most significant form of public-art patronage in France during those years." We can only imagine how quickly Rousseau's designs got shot down - and ironically, how many tourists would now be trooping to any provincial town whose officials had been clairvoyant enough to commission a mural by Henri Rousseau!

Does this mean there's hope for our Sunday painter, even though his attempts at perspective give the viewer a migraine, and his renderings of the human body are, to quote Cole Porter, "less than Greek"? Perhaps - if he is immensely gifted, incredibly tenacious, and capable of following Rousseau's example, which was to find a way around his weaknesses, build on his strengths, and manage to live in the right place at the right time.

Take anatomy. Rousseau himself seems to have been nicely put together (by all reports, the ladies of his acquaintance thought so). But the same cannot be said of the human figures he painted. It is hard to attach any real artistic merit to his stabs at the formal black-suited portrait, a genre for which the standard was set back in 1524, by Titian's Man With a Glove. At best, Rousseau spoofs the genre, introducing such bizarre elements as the striped cat, red fez, and itty-bitty smokestacks in Portrait of Monsieur X (Pierre Loti) (1910). But at worst, his portraits bear more than a passing resemblance to the stiff, labored ancestor pictures that stare grimly from the walls of every preserved colonial homestead in America.

But Rousseau got around this problem by the ingenious device of dressing his human figures in fancy costumes and reducing them to decorative motifs, preferably in landscapes where his other weakness, a lack of perspective, had already been solved. In The Banquet Years, Roger Shattuck's superb study of fin-de-siècle Parisian culture, he compiles a list of Rousseau's finest works, including four jungle pictures (about which more below) and the stunning Carnival Evening (1886). (Full disclosure: Carnival Evening is not only my favorite Rousseau, it is also one of my favorite paintings, period.)

It's fascinating to see how this early canvas prefigures Rousseau's later triumphs. Rather than struggle to make his pair of carnival-goers lifelike, he transforms them into exquisite little dolls, dressed in pale garments that glow as luminously against the bare, black trees as the full moon and delicate clouds glow against the twilit sky. And strikingly, there is no perspective to speak of, because the line of the horizon has been brought as low as possible - suggesting a ridge, or the crest of a hill. Much has been made of the tiny grimacing face attached to the gazebo on the left, but that is not what makes this picture so haunting. Rather it is the dropped horizon, which creates an eerie emptiness behind the figures and the dark woods that loom at their back.

Ever since the artists of the fourteenth century read the Latin translation of Ibn al-Haytham's tenth-century treatise, Optics, Western painting has valued pictorial depth, or the illusion of three-dimensional space on a two-dimensional surface. The calculations required to create this illusion have evolved from Giotto's rudimentary algebra to the elaborate ratios worked out by Erwin Panofsky. But through it all, any malfunction of approved perspective has been called crude and backward. Obviously there's nothing crude or backward about great works created by civilizations that did not study perspective: medieval manuscripts, Chinese paintings, and Persian miniatures, to name a few. But these have, at times, been called decorative - or as the phrase goes when the intention is to disparage, merely decorative. In Rousseau's day, Cezanne's dismantling of perspective was being eagerly taken up by the cubists and other modernists. But that didn't stop them from condescending to Rousseau. After all, playing with or even subverting classical perspective was one thing; being totally incapable of using it was another.

But here, too, Rousseau proved resourceful. Rather than wrestle blindly with sight lines and vanishing points, his best paintings simply abandon the whole enchilada and find other ways to evoke a sense of distance. For instance, in most of the jungle paintings, depth is suggested either by color, with the greens in the background darker and more saturated than those in the foreground; or by scale, with fruits and flowers getting smaller the farther they recede.

And sometime he uses neither. In Fight Between a Tiger and a Buffalo (1908), the colors are uniform, and despite a faint hint of horizon, the space is extremely shallow. Indeed, the oranges in the grass in front of the struggling beasts are the same size as those in the trees behind them. The bananas get bigger, but they do so from left to right, not from front to back - no doubt because Rousseau wanted to balance out what is essentially a flat composition. Are the best of the jungle pictures - Tiger in a Tropical Storm (Surprised) (1891), The Hungry Lion Throws itself onto the Antelope (1905), The Merry Jesters (1906), and The Snake Charmer (1907) - decorative? You bet. Does that make them inferior to umpteen thousand academic paintings in which fully sculpted nudes writhe in deep illusionistic space? No way.

As this exhibition makes abundantly clear, Rousseau's images of tropical flora, fauna, and humanity came from popular magazines, travel books, the Paris botanical garden and zoo, and the 1889 World's Fair, which featured 44 different ethnic and historical pavilions surrounding the base of the brand-new Eiffel Tower. Even at the time, it was obvious that Rousseau was taking a free hand, placing a Mongolian deer in the jungle (The Waterfall, 1910) and conjuring encounters not found in nature (Tropical Landscape - An American Indian Struggling with a Gorilla, 1910). But as Christopher Green writes in the catalogue, Rousseau's real accomplishment was to create a dream jungle: "a theater of fears and desires" for his fellow bourgeois Parisians.

It's nice to know that, after all those years of rejection and poverty, Rousseau finally made it big. His last jungle painting, The Dream (1910) is right up there with Leonardo's Mona Lisa and Van Gogh's Sunflowers as one of art's All-Time Greatest Hits. Certainly the National Gallery show is organized around this happy ending, with The Dream hung at the very end, where it attracts countless oohs, ahs, and contented sighs. ("See the elephant, Josh?" "Look, Melissa, is that a tiger's tail or a snake?" "Yes, we can go for ice cream now.")

Too bad The Dream is not the culmination it is chalked up to be. It is weirdly academic, in the sense that it is crammed with everything you could possibly want, all the goodies in Rousseau's bag of tricks, plus a nude. But it is not thrilling and (dare I say it?) not beautiful. If you want to see Rousseau's most beautiful jungle painting, go back to the first, Tiger in a Tropical Storm (Surprised), painted nineteen years earlier. (It is no accident that the curators put that one on the cover of the catalogue.) Compare that fabulous canvas, full of gorgeous color, movement, and pattern, with the formulaic jungle paintings in the room just before The Dream, and you'll see a definite decline. The painful truth is that Rousseau's career peaked early, with a handful of astonishing canvases, in which he overcame his technical limitations and expressed something urgent and ineffable that had clearly been bottled up in his soul. By the time he painted The Dream, success had turned to formula, and he was, as they say, churning them out.

Question for a Sunday painter: How do you keep a dream alive after it has come true?

(This review appeared first in the Weekly Standard.)