Usually attributed to that prolific aphorist Anonymous, the sadly true notion that "Happiness writes white" probably emerged from Tin Pan Alley or Broadway, when somebody finally realised why most of the good love songs were about love lost. The idea might seem to conform to the standard Romantic conception of poetry, but it is important to remember that the Romantic conception was a real discovery of something that had always been true: art is an outward integration inspired by the artist's inner disintegration.

The converse holds. Contentment has either no need of artistic expression, or few resources for it. Even when we doubt this is so, we think it ought to be so, and apply the concept to other arts as well as to poetry. One of the reasons we speak so slightingly of Mendelssohn, when we put him beside Beethoven, is that we can hear Mendelssohn's music smiling - or we believe we can, which is enough to make us patronise him, reserving our unmixed approval for Beethoven, even when the personality revealed in Beethoven's life story strikes us as actively unpleasant.

With a less glaring discrepancy, but in the same way, our opinion of Renoir will always be lower than our opinion of Monet, because Renoir suffered less. Renoir might have suffered more had he not been so reasonable, but he gets no points for that. Renoir correctly found Monet irresponsible and loathed the way he spread misery among his women, but the more that the truth of their lives came out, the more that Monet looked serious vis-a-vis Renoir, until, by now, Renoir's reputation carries an indelible question mark. Instead of putting himself to the anxieties of developing his manière aigre, he might have done better studying to be more miserable.

Unhappy artists are to be pitied, but often not for the apparent cause of their unhappiness, which they might have arranged. If they had become artists in order to deal with a psychic imbalance that was implanted early, the urge to remain productive might well entail a deliberate avoidance of ordinary happiness, especially in its domestic branch. When Philip Larkin said "deprivation is for me what daffodils were for Wordsworth", he was giving a powerful hint that he would stay deprived if he could. Eliot showed the cost of settling for bliss: it produced exactly one late, pale lyric, whereas the opposite had produced The Waste Land.

Yeats probably knew he was a fool for love. Knowing it, he was no fool at all, but perhaps something even more pitiable, an artist so dedicated that he would hoodwink reality if it threatened to make him content. His instruction to himself -"Never give all the heart" - would have killed his work had he obeyed it. He always gave all the heart, over and over: but only to those women who shared the useful characteristic of incomprehension. His wife, George, paid the penalty for understanding him: she inspired him to gratitude, but seldom to a rhapsody.

Yeats's muse, on the other hand, whoever she might happen currently to be, fled forever out of reach, like a Daphne just a touch faster on her feet than Apollo. In Norman Jeffares' excellent Commentary on the Collected Poems of W. B. Yeats, we find Yeats expressing himself thus: "How much of the best I have done and still do is but the attempt to explain myself to her? If she understood I should lack a reason for writing, and one can never have too many reasons for doing what is so laborious." He means that he wouldn't write poetry unless he had to, and implies that nothing must be allowed to remove the reason. Chateaubriand, in the preface to Atala, said that when the Muses cry, it is only to look more beautiful. The immediate implication was that the poet might not be beyond courting some high-quality misery in order to make the Muses tearful.

There is a deeper implication: the poet will exploit grief when it comes. Peter Porter, with characteristic frankness, laid bare this truth in the sequence of monumentally beautiful poems he wrote in commemoration of his first wife. But true artists don't need love trouble to stave off happiness: all they have to do is look at the world. In that regard, all the great art we know of carries within its compass a guarantee that its creator is not content. Shakespeare's sonnets are the most convincing possible assertion that love is not only a fine thing but that we have scarcely lived if we are shut out of it. Yet all the ecstasy in the sonnets would amount to nothing if it were not threatened by time and death, which he evokes with at least the invention that he lavishes on the erotic. Similarly, Dante's Inferno might be hard to take if we didn't know that he would later write the Paradiso, but the Paradiso would be unbearable without the Inferno. It would just be a hyperbolic brochure for spiritual fulfilment.

We don't allow our master artists to be merely twee. But neither, on the whole, do they themselves; and they, as we, will always put Watteau, Boucher and Fragonard in the second rank merely for suggesting that Arcadia can be an unmixed pleasure and go on forever; although it could be said that the melancholy of monarchical absolutism imposing its requirements somehow seeps through to give even manufactured bliss an undertone of menace. Besides, Fragonard's pretty girl reading her book will grow old and die: we know that, and we know he knows that, or he would not have been so seized with the beauty of her concentration. French painting and music are copious with what we would call the merely lovely if we were not so confident about bringing our own unhappiness to the picture or the composition, even when French painting and music are careful to avoid any of their own.

We can be equally confident that their makers would never have shown that care if they themselves had been truly untroubled. To be undisturbed and yet still creative would be impossible. When playing Vermeer, Colin Firth looked grief-stricken but probably underplayed it. To want to produce that degree of serenity is a sign (for once the semiotic vocabulary becomes appropriate) of a sensitivity to turbulence that can't, as it were, be brushed away. If the world's horror had not been eating at the artist's soul, we would never have seen the girl with the pearl earring. The instinct, on the part of those who project utopia, to leave the artists out of it is politically deplorable but aesthetically sound: they would produce anodyne art.