Philip Hensher: As a playwright, he is a fine diarist

Usually, when one says of a play that one has mixed feelings about it, it means the opposite; very unmixed feelings of disdain and dislike. In the case of the revival of Simon Gray's 1975 play Otherwise Engaged, which opens at the Criterion in London next week, the expression is exact. One hardly knows what to wish for it; even someone who, like me, hugely admires and enjoys Gray's writing may contemplate the prospect of the production turning out to be a success with perhaps unworthy doubts.

Gray has had a peculiar career. He is the sort of playwright one wishes there were more of: an intellectual boulevardier, taking on serious subjects with a light touch, his wit reflecting an amused, tolerant attitude. There is indignation in the plays, and rage, but really, Gray is one of those Englishmen who may from time to time attempt philosophy, but who finds cheerfulness breaking in.

The plays are always likeable, but have had bad luck. The enjoyment audiences have had out of Butley, Quartermaine's Terms, The Common Pursuit and many others have only periodically translated into large-scale success. Gray may even be best known for having written the (excellent) play Cell Mates, from which the ghastly Stephen Fry ran off in a state of nervous breakdown; the play had to be taken off soon after.

In addition to bad luck and the periodic but venomously expressed opinions of critics, Gray has had some fairly extravagant disasters in his private life, notoriously pursuing a four-bottle-a-day champagne habit to the brink of death before giving up entirely. In possession of a lot of stories of catastrophes, professional and private, and somewhat disconsolate about his career as a playwright, he started to publish what he calls "diaries", though they are, in reality, Shandyesque ramblings.

These now add up to half a dozen volumes or so, and they have increasingly come to combine reflections on the immediate surroundings as Gray is writing, memories of childhood and later - crucially - some killingly funny, unforgiving stories of disaster, mostly theatrical. Enter a Fox was mostly about a production going wrong; Fat Chance a stupendously unforgiving (and who can blame him?) account of the debacle provoked by Fry.

The diaries have gone on dwelling on writerly embarrassments and flops, but, oddly, they have started to become rather popular and, certainly, universally acclaimed. The Smoking Diaries last year, an extraordinarily original and excruciatingly funny book, was received with the sort of rapture rarely accorded to his plays. The Year of the Jouncer, published early next year, returns to the theme with an account of a theatrical catastrophe as Gray receives a drubbing from the critics. The volume covers the period when The Smoking Diaries comes out, too; but with that, with an unarguable popular success, Gray can, paradoxically, do very little. His theme is a writer's failure; and for his writing to be successful, it needs to sustain a sense of its own failure elsewhere.

Of course, one would love to see Otherwise Engaged succeed, and be followed by hugely successful new plays. But the awkward fact is that the plays, good as they are, have become less important than what initially seemed an incidental activity, the extraordinary, unique style of the diaries. And one can't see what the diaries would do with the experience of vast success; their enraged subject is failure, and one wonders whether they could do without it.

It's an odd phenomenon, the artist who ends up making his mark with a part of his work he himself hardly values. Lorenzo da Ponte is immortal through his librettos for Mozart, and only that; he himself hardly mentions Mozart in the course of a long autobiography. The same is true for Boito, remembered not as a composer but as Verdi's librettist, and perhaps even of Hofmannsthal.

Hogarth wanted to be remembered for his unbelievably awful history paintings; Evelyn Waugh said that Helena was his best novel; Scott Joplin thought his importance to posterity would lie in Treemonisha. It isn't unusual. The most awful story, perhaps, is that of Benjamin Haydon, an academic painter of the Romantic period. He believed himself a genius, neglected by contemporaries because of their poor taste: in fact, he was an inept, talentless painter.

But we know what he thought, because he set it all down in an incomparable diary. Haydon's is an extreme case, because he really was a genius - a genius of a writer. He had no idea, and went on pinning his hopes on his awful painting.

Gray really is an underestimated, admirable playwright, yet he starts to look much more like a great, lasting writer in something unexpected, the incidental volumes of prose musings. You can't choose what you will be remembered by; it is as well to accept that it may prove to be something utterly infuriating, or it might be nothing at all.