Stewart himself accepts this judgement with the same rueful, deprecating tone he adopts whenever he discusses his second self: the crime-writer Michael Innes, author of some 40 novels and volumes of short stories, and creator of Inspector (later Sir John) Appleby. Others have tended to endorse his opinion. Julian Symons, in his authoritative study of crime fiction, Bloody Murder, pigeon-holes him as 'the finest of the Farceurs, a writer who turns the detective story into an over-civilised joke'.
Both Stewart's and Symons's points of view are easily understandable. Symons' preference is for the grimy psychological crime novel, and Innes displays a taste for the grotesque and outlandish - intelligent horses and multiple personality disorders are grist to his mill. As a novelist in his own right (his most notable work is the quintet A Staircase in Surrey) and a distinguished academic (author of the final volume of the Oxford History of English Literature), it's not unnatural that Stewart should be saddened when his achievements are overshadowed by these prankish outpourings. Both are missing something.
Stewart says that he started on detective novels because he stood in awe of 'proper novels'. Because he never took them seriously, at their best - in The Journeying Boy, Stop Press, Lament for a Maker - the Innes books have an air of unchecked intellectual vigour missing from most detective fiction.
Writing of John Buchan (an acknowledged inspiration for Innes), Allan Massie has said that his Hannay stories go deeper than their author knew or intended. The same is true of the Appleby novels. Innes may not have managed a Treasure Island - though in Appleby on Ararat he takes a poke at Coral Island - but the shadow of Robert Louis Stevenson lies over him. Like Stevenson, he wrote genre fiction that broaches larger themes, the playfulness of the writing disguising the seriousness of the concerns.
Over and over, he reveals a preoccupation with the relation of a writer to his creations, and with problems of identity. He experiments with the mechanisms of imposture, repeatedly suggesting, though never out loud, that we will accept impostors for the puniest reasons - the puniest of all being physical appearance. Twins and doubles recur, hinting at an obsession with duality reminiscent of Stevenson's Dr Jekyll and Mr Hyde or The Master of Ballantrae.
He has faults. Many of his books suffer from over-elaborate plots and over-literary dialogue; some later books are repetitive and constructed on flimsy foundations (Sheikhs and Adders is purely an excuse for a dull pun); and he's a dreadful intellectual snob. But at his best there's an unforced erudition, pure glee and shrewd human insight.