For one thing, some people become critics precisely because it is the quickest or most expedient means of becoming an artist. Take the case of the French nouvelle vague directors of the late Fifties. All of these men - Francois Truffaut, Jean-Luc Godard, Claude Chabrol, Jacques Rivette, Eric Rohmer and Jacques Doniol-Valcroze - had begun their film careers by writing feisty, impassioned, pretentious, lyrical and downright cranky critical screeds in the pages of the journal Cahiers du Cinema and other publications; within the space of a single year from l958 to 1959, they had all made the transition to directing (although one of them, Godard, would go on to explain that he had never given up being a critic: his movies were simply works of criticism composed on celluloid).
It was a terrific coup, but, when you ponder it a little, a wholly logical one. Cinema, the most industrial of all the arts, requires huge sums of money for even the most modest productions; while an impoverished young poet can scratch out sestinas in his bedsit, a young director needs to con or charm his elders into handing over thousands. What more efficient way into the game than shouting loudly, in the pages of a journal that is read in all the right places, that you and you alone know how things should really be done? More modestly, what better way of learning the rudiments of your chosen art than by finding a job that lets you engage with it day by day, and helps you meet the older artists who will be your best teachers?
One account of the movement describes the nouvelle vague's achievement as "unparalleled in the history of cinema and perhaps all the other arts": but it is really only the spectacular nature of its success that divides this act of critical insurgency from dozens of others. George Bernard Shaw took the stage after having made his name with a dazzling career in theatrical and musical criticism, some of which (The Quintessence of Ibsenism, for example) now reads more happily than a lot of his later dramatic prose. Neil Tennant, to take a less canonical instance, founded the Pet Shop Boys after writing professionally about pop music and realising that, yes sir, he could boogie; Chrissie Hynde, a journalist with New Musical Express in the early 1970s, went on to form the Pretenders; while the film critic Anne Billson has recently turned to writing horror novels such as The Vampiric Suckers.
More common still than the artists who begin their careers as critics is the phenomenon of ambitious young artists obliged to moonlight as critics to advance their causes to the public. (Works of art may indeed create the tastes by which they are understood, but sometimes they need a bit of a shove in print.) In large measure, the history of modern art is the history of manifestos: wherever there is a new "ism" in the arts, you will find either that the gang of artists have picked up a tame critic or two to explain and publicise what they are up to, or (more usually) that they have set to and done the job themselves.
Hence the pugnacious writings of Marinetti & co, of Le Corbusier, of Wagner, or of Ezra Pound and Wyndham Lewis in the service of Vorticism. Both of these last two are interesting in other ways. Pound also wrote criticism - acre upon acre of the stuff - about literature, art and music, simply to pay the bills; Lewis, who began blasting away in the pages of fly-by-night publications that seldom ran to their third issue, was obliged by his bank manager to spend the last years of his professional life as the art critic for The Listener, until his failing eyesight disqualified him. The other reason why so many artists of the 20th century have turned critic is not, pace Mr Craig's sly dig, that they have conspicuously failed as creators, but that our economies are so arranged that it is, on the whole, easier to earn a living as some kind of journalist than as some kind of artist.
Lurking not so deep beneath the Nicholas Craig line is a post-Romantic notion about the necessary separation between the creative and critical faculties; and the English Romantic movement seems to offer us the exemplary warning in the life of Coleridge, who seems to have abandoned his astonishing poetic gifts in favour of criticism and metaphysics. Even TS Eliot, by many reckonings the outstanding poacher-gamekeeper of the century, could make a wry allusion to the "sad ghost of Coleridge''.
It's plainly the case that there have been many outstanding critics who have never practised that art of which they preached, or did so only once or twice, with embarrassing results. It's no less obviously the case that most of the best critics of the arts have also been its best practitioners. Hence Alberti, Leonardo da Vinci, Sir Joshua Reynolds, Hogarth, Blake, Turner and Klee; hence Dryden, Johnson, Keats, Henry James and the late Donald Davie; hence, to change the rules of engagement slightly, all the poets from Baudelaire to John Ashbery who have written art criticism. This is not to deny that many critics have indeed been the ignorant, graceless hacks of defensive artistic mythology; but it is to underline the validity of a rather less sentimental view. To adapt, again, from a mordant saying of TS Eliot: most critics are failed artists, and so are most artists.