Une comte d'un con et de M Lacan, le troisième

It's often said that the model for Courbet’s painting was Joanna Hiffernan. The lover of James Whistler (and others), Jo was an Irishwoman with long, flaming red hair - which, yes, does conflict with the colour and texture of the hair we see in the painting. She is, though, certainly the woman in both Courbet's La belle Irlandaise (Portrait of Jo) - which is in the Met in New York - and in Whistler's Symphony in White, No 1: The White Girl - in the National Gallery, Washington.

But in truth, given the evidence before your eyes, Jo is an unlikely candidate. An improbable one, in fact. Rather, the painting's extreme, 19th century realism - hyper-realism before its time, even - is not just an extension of Courbet's general pursuit of the real and everyday. It's clearly photographic in composition, tone, colour balance, size even - it's far, far smaller than Courbet's usual giant canvases.

More than that, there's a photograph that is a more-than-fair candidate for its, well, origin. It's by one of the first photographers of pornography - or, at least, of naked female bodies, with the aim of awakening male (and perhaps female) imaginations. Though he traded under the name Billon, he was in fact the Auguste Belloc, the great, innovatory photographer of mid-19th century Paris. He photographed the great and good of the Second Empire. He invented the wet collodion process - which Sally Mann has recently re-put to good purpose photographing her children naked.

(The reason we know Billon and Belloc were the same man, by the was, is because the forward-looking but statist French state had decided that photographs should have the same copyright status as paintings etc. So photographers established their rights by sending copy prints to a national agency - which held them in store, whether they were of great men's bearded faces or young women's unshaven vulvas. Oh, and there is no link at all between Auguste Belloc and his near namesake and fellow photographer of unclothed women, EJ Bellocq who captured the pre-First World War New Orleans brothel world that gave us jazz, probably, and Jelly Roll Morton, certainly.)

The particular image that Courbet most likely used as a basis was one of a series of hand-coloured stereoscopes. There is the same pale-skinned, generously haired young woman, the same headlessness and rumpled white linen and a very similar pose. It's true, though, that Courbet removed the stockings and added breasts — well, an edge of one.

It was the mid-1860s, photography had only been around a few years and already there was both colour and 3D. (I'd have put an exclamation mark at the end of that sentence if I didn't have a lifelong aversion to their, inevitably cheap, excitement.) The first photograph of a (clothed) human was taken in 1838. In 1860, a police raid on Belloc's stock netted 4,000 erotic images. As so often, the desire to satisfy human desire is a motor for invention. Now, if only we could find a way to monetise it sexually, the solution to global warming would be round the corner.

Next up A Turkish gentleman makes a proposition to a French painter.