Ever since the graffito that manifested itself on the plaster of the wall at Belshazzar's feast, hoisted words, banner words, have claimed our attention in one form or another. Incised on monuments and memorials, they acquire a weightiness that seeks to imply durable worth; as mottos scrolled on flags, they command loyalty; foot-high on advertising hoardings and billboards, they act the courtesan or the sergeant-major, out to charm or bully; in the tags and aphorisms of graffiti artists they decorate grey cities of means with colourful ends suggestive of a different order. More transiently, they appear and flick away again in soundbites and shoals on the screens of our cinemas and televisions.

A number of them settled on the walls of the Bonnard exhibition at the London Tate Gallery earlier this year, by the entrance. They included the artist's assertion that 'in the subtle equilibrium between lies and truth, everything is relative, everything is a matter of more or less'. From which he concluded that 'extreme sincerity runs the risk of appearing either ridiculous or untenable'.

Does the observation earn its highlighting, its place on the wall, especially loaded as it is by the qualifying adjective 'extreme'? It's certainly the case that the matter of sincerity has occupied writers as well as painters considerably over the years, from Bacon, as when writing 'Of Simulation and Dissimulation', to Hazlitt, for instance in his essay on 'Good-Nature'; from Wilde, whether in 'The Critic as Artist' ('A little ...