This painting hangs in the mind's eye: a philosopher in a blue coat climbs up into a tree whose branches and leaves hang like a tattered map. He moves gingerly among the cities and small republics, his fingers inching over fluttering islands, the pollen of snow-capped peaks. Cherries stop him and he begins to pick them, throwing them down to two girls laughing on an ocean of grass. Rousseau - for it is he - is happy amid the cherry blossom and the cherries whose redness suggests the redness of lips. 'Why', he asks himself, 'are my lips not cherries?' So cherry lips rain down from the tree, lips laughing into laps held out to receive the gifts of juice and flesh. Lips and laps and cherries and hanging above the little scene the blue of the philosopher's coat whose folds and colour have the empyrean quality a painter sweated for, putting sky in clothes. This is cherry idyll, 'L'idylle des cerises'. But it is not a painting. It is an anecdote from Rousseau's Confessions and here and there, stray details, like traces of the brush's tip, make you pause and look or listen more carefully. For Mlle Galley and Mlle de Graffenried don't just catch the cherries, they throw the stones back up to Rousseau. Which means they eat them first, detach the soft flesh with their tongues and lips, spit out the stones then cast them back at the tree-hung philosopher. If you've eaten ...