If someone were to ask you to name the most important painting of the twentieth century, which one would you choose? Guernica? Les Demoiselles d'Avignon? Something by Pollock, perhaps? Almost certainly it would be something from the first fifty years of the century, when painting still ruled the world of art. Very well, I'll admit, such a question is not only difficult, perhaps even impossible, certainly unfair, but also good for a long, heated argument if posed in the right group of people. Very well then, let me ask a somewhat easier one. What do you think was the most important painting of the nineteenth century? You're probably asking, how is that any less difficult? In fact, there was far more important painting done during the nineteenth century than the twentieth. It was the century of the painter, when painting reached its zenith as a communicative art form. It was a century that may have seen more working painters and more outstanding painters than any before or since - Michelangelo, Caravaggio, Rembrandt, Picasso, and Jim Lane notwithstanding. It was a century that began with David and ended with Cézanne. It was the century of Ingres, of Courbet, of Manet, of Monet, of van Gogh, and of Georges Pierre Seurat.

I would choose A Sunday on La Grande Jatte by the ever patient Monsieur Seurat of Paris, France. Notice here I'm not saying it's the best painting of the century (although it is a superb piece of work) nor is it the most beautiful (although it is, in fact, quite stunning). It hangs today in the Art Institute of Chicago and, along with American Gothic and The Night Hawks, competes in being their most famous work. Of the three, it's certainly the most massive, weighing in at some seven feet tall and ten feet wide; and, if the good old Yankee work ethic means anything, it easily wins as the most laborious work since Michelangelo took up ceiling decoration. Its size is the direct result of a bad experience Seurat had with a similar, earlier work, Bathing at Asnieres, which was so radical and disconcerting at the time, his fellow disconcerting radicals who hung Salon des Refusés ended up placing it behind a door in a poorly lit side room of the main Salon.

Not only was La Grande Jatte too big to hide behind most doors in Paris, it was too big to be ignored by the Paris art world. If Impressionism had shaken up the art world of its day, this epic work shook up Impressionism. Theoretically, it was Impressionism made law. It was Impressionism laid down in red, yellow, blue, white, and (gasp) black. But it was such a departure from the spirit of impressionism, the in plein air, natural, fresh, spontaneity of the movement, that it was, in fact, a whole new movement, which has since come to be known as Neo-Impressionism (not to be confused with the broader Post-Impressionism of which it was merely the first step). It had in common with the Impressionism of its day its rather mundane subject matter - a sunny Sunday afternoon in the park. And, despite Seurat's having overcome the Impressionists' allergy to black, it reflected prevailing Impressionist colour theory. But there the similarities ended. Whereas most Impressionist works were modest, one-sitting, portable easel paintings, this sucker was mural size and took just over two years to produce (working night and day at that)!

Despite his secretive, workaholic devotion to his art, Seurat painted only about a dozen works (he died young, at the age of 32). Yet, primarily with this one work (by far his best), he managed to change the whole direction of art, not so much in his own century but in the one to follow. The phrase hadn't even been invented yet, but this could well be considered the first "art for art's sake" painting in the history of art. Though the artist was fond of the park and visited it often in his childhood, as well as in making preliminary studies (which the Art Institute proudly displays), the subject was merely a source of design inspiration - a means to an end. It communicates little about its time and place other than the high fashion and the sedate relaxation of the Parisian populace. Instead, it is the shimmering, dotted, divisionalism or Pointillism (as it's more popularly called) that was its reason for existence. In fact, it was a science experiment in painting with forms simplified, abstracted, and sublimated to the cause of colour theory, mass, space, balance, and visual texture.

Few paintings of its century could claim such a breakthrough, and none could claim to have explored what amounted to a whole new type of painting so deeply or so completely. Yet strangely, Seurat inspired few direct followers other than his colleague, Paul Signac (whose contribution was to move Pointillism still further from relative objectivity). Seurat's true followers were not Pointillists (the technique was too laborious to be economically viable). His true followers were artists such as Matisse, Picasso, Kandinsky, Miró, and others in the twentieth century who, while digesting his analytic exploration of colour, drew their most profound influence from his willingness to elevate various design elements to dominance in their work, leading ultimately to the exclusion of subjective content. A Sunday on La Grande Jatte was painted between the years 1884 and 1886. It was a natural, evolutionary step following Impressionism, but at the same time, in its importance as a harbinger of twentieth century art, it was at least twenty years ahead of its time. If for no other reason, that makes it, in my view, the most important single painting of the nineteenth century.