He did a good deal of it himself, both as a BBC music producer and as an intellectual celebrity who seems to have memorised every note of Haydn's string quartets and virtually owned Schoenberg. He was not only the prophet of the immortals, either, for Britten dedicated his third string quartet to him. After retirement, Keller wrote for the late, lamented Listener, castigating bran-tub sequences such as Mainly for Pleasure (precursor of In Tune) on Radio 3 while unblushingly praising not just the programmes he approved of, but also their producers. Keller's real importance, though, was in showing how music worked, and writers on the subject still refer to him 12 years after his death.
One of the first to give public lectures explaining the art was the Viennese critic Edward Hanslick, the model for the nit-picking, hide-bound Beckmesser in Wagner's Die Meistersinger. Yet, as Donald Macleod pointed out in Monday's programme, Hanslick wrote a generous obituary of Wagner, which is more than the egocentric and famously mean-spirited Wagner would ever have done for him.
Hanslick has been cast as the archetypal poison-pen critic, opposing anything progressive. His silliest criticisms were levelled at Tchaikovsky, whose Violin Concerto "stank", he wrote, and who would have re-written the five-in-a-bar waltz in the Pathetique Symphony in a more regular metre. Yet Hanslick can hardly be blamed for finding some of Richard Strauss's earlier music exaggerated and tasteless - many still think that - and he became opposed to Wagner only as the latter developed the idea of seamless music drama, dissolving the old formal divisions in opera.
Wagner's music was still contentious during Debussy's periodic stints of music criticism, outlined on Tuesday. Whereas Beethoven's writing for the orchestra was black and white, allowing for the most interesting gradations of grey in between (he wrote), Wagner's was a sort of multi- coloured putty that obscured the individual character of instruments, so that it became hard to tell strings from trombones. Which sounds very much like a question of taste - you could just as well praise Wagner for wonderful blending.
Debussy inveighed against pale French imitations of Wagner precisely because he fought so hard not to become one himself. For him, Wagner was an artistic father-figure who had to be, if not buried, then at least superseded. And while never admitting any influence from Wagner, he had no hesitation in telling someone about to see Mussorgsky's Boris Godunov that the whole of his own Pelleas was there.
Mussorgsky was one of Debussy's great causes, not least because he was hardly known, even in Russia, and Debussy - always the iconoclast - likened him to "an inquisitive savage who discovers music for the first time". As criticism - heightening our awareness or pinpointing what we may already have felt - that is brilliant. But still more thrilling is the fact that Debussy, one of the determining forces of music's last hundred years, said it, and so revealed something of himself.