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What we recognise as modern art has one outstanding characteristic: it is not mimetic. That is, it does not seek to create an illusionistic representation of the visible world but rather to establish its own reality as an independent object - its autonomy, in the jargon. With this goes a corresponding emphasis on the purely formal or aesthetic aspect of the work - its character as an autonomous structure of line, form, colour, texture. In modern art, everyday reality may be referred to, or evoked, in ways ranging from more or less distorted or stylised representations to the direct incorporation in the work of "real" objects or materials. Or it may be excluded altogether in favour of some form of abstraction - perhaps the most purely modern art.

Paradoxically, the evolution of art towards this condition can be traced back to the 19th century rejection of the Renaissance tradition of "history" or "high art", by then largely debased in the hands of the academies, in favour of a direct engagement with the real world, especially nature.

John Constable is as good a place as any to locate a beginning of this process: in 1828, he wrote bitterly of those who preferred the shaggy posteriors of a satyr to the moral feeling of landscape. He was referring to the members of the Royal Academy who had just failed to elect him to the Academy in favour of William Etty, a painter of "high art" pictures, the ostensible moral content of which, or simply their use of high art motifs (eg nymphs and satyrs), screened their real salaciousness.

Constable's comment evinces a crucial aspect of modern art - its claiming of the moral high ground, initially for an art based on the truth of nature or the everyday realities of life. Later, in the 20th century, art claimed this moral eminence, precisely in the degree to which it was not an illusion and could be seen as an embodiment or emblem of truth - because it was true to itself as a medium and true also to the artist's personal vision, unsullied by the demands of patrons or, indeed, any material considerations.

Let's return to the apparent paradox of a line from Constable's rural landscapes to, say, Mondrian blocks of primary colours. Constable's whole practice was based on working direct from nature; yet rather than producing a smooth, illusionistic image, he found ways to represent what he saw in marks of paint that had a real, physical, anti-illusionistic presence. His contemporaries in England (but not in France, where he was admired) were completely baffled by this, together with the apparent artlessness, the lack of reference to tradition, of his approach to the motif: "My art flatters nobody by imitation, it courts nobody by smoothness, it tickles nobody by petiteness ... how then can I hope to be popular?"

Constable had no successors in England, but in France later in the century the artists who became known as the Impressionists took up the radical practice (instigated by Constable) of painting a whole "finished" picture out of doors. The result seems to have been an increasing focus on the motif as a pattern of light and colour, an increasing emphasis on the brushstroke, and an increasing degree of abstraction (as, for example, in Monet's Rouen Cathedral series or his waterlilies).

From the mid-1880s the so-called Post-Impressionists - Van Gogh, Gauguin, Seurat, Cezanne - took Impressionism in various different directions but consistently emphasised pattern and heightened or exaggerated colour. In 1905, the Fauve group, led by Matisse, startled Paris with paintings in which colour appeared entirely detached from observed reality and in which the motif was rendered, literally, in the broadest brush terms. It remained for the Cubists, Picasso and Braque, a year or so later, to dismember the motif itself and set the scene for the emergence of the pure geometric abstraction of Mondrian and the Russian Malevich, about 1915-20. Inspired by Monet, another Russian, Wassily Kandinsky had, about 1910, evolved a free-form kind of abstraction.

The development of modern art coincided with that of photography. One intriguing view is that in order to preserve the uniqueness of their art (and its value in the market) painters simply had to make it as distinct from photography as possible.

After the Second World War, attempts were made to put modern art into a theoretical strait-jacket, notably by the American critic Clement Greenberg, in the context of the post-war American art known as Abstract Expressionism and its immediate successors. In the Seventies the term "Post-Modernism" was coined to define new art which appeared to ignore or reject the concerns of "classic" modernism. Such art openly embraced popular or commercial culture, and freely plundered the art of the past, recycling its imagery.

Critical debate has raged increasingly ever since. Meanwhile, art sails serenely on ...

Tomorrow: Opera