Cities are stitched out of prohibitions, zones from which we prefer to avert our eyes. They are there, we know they are there; we prefer not to witness them. Autopsy fridges, tiled cells drenched with blood and gristle, featherless chickens packed into cages (all these things will, in time, feature as part of the art/industry business. Calibrated shock effects sponsored – and delivered – by millionaires. They underwrite the ‘rescue’ of de-energized zones such as Hoxton and Shoreditch). Naked poverty is the final taboo. Civilisation depends on elective amnesia: gaudy postcards, intended for tourists and day-trippers, depict public buildings, sanitised royalty (the deader, the better), scarlet buses, uniforms. But it remains a perversion of that card-carrying voyeur, the professional photographer, to stick a nose into the most blighted areas: murder scenes, traffic accidents, opium dens. Vistas of sweated labour that hover between an affront to our sense of decency and an occasion for sado-masochistic relish. Arthur Munby, a ‘respectable’ Victorian barrister, was obsessed by working women: he paid them to pose in their grimy clothes, faces blackened. He married his servant, Hannah Culwick, and photographed her, at leisure, as she scrubbed and skivvied. Heavy features, heavy boots.

From the start, hobbled by cumbersome equipment, image-snoops have franchised discriminations of horror and shame, dowsing for heat in the nooks and crannies from which ordinary citizens are barred. Dens and brothels and gaming houses in which both punters and operators insist upon anonymity: no name, no face. Watch, fuck, pay. There is a particular quality, therefore, to faces unwillingly (or unknowingly) exposed. A more savage pornography than the awkwardly splayed crevices of ‘art’ postcards lost in sepia. A flash of magnesium. A frying-pan filled with the stuff: blinded. Vagrants, psychopaths and off-the-book labourers. The illegitimate, disenfranchised and exploited: prisoners of the metropolis. Take a magnifying glass to those historic prints: shame, terror, disorientation. The rictus. Photographers, from Jacob Riis to Weegee, specialised in it. The film-maker, Stan Brakhage, moving on from celebrations of love-making, childbirth, the passage of light, realised that a city is best understood through its secret rituals: forensic, reserved for initiates. He made three films in Pittsburgh: night patrol car, open-heart surgery, autopsy slab. The heart of things exposed to his trembling lens.

Out of prohibition and anxiety, that tension, comes vision. Photography as an act of restitution. We will look at framed prints of things we walk miles to avoid: drinking schools, transvestite prostitutes, mad folk gambolling on the grass of an asylum.

A key exhibit would be Jack London’s lower depths travelogue, The People of the Abyss, in its October 1903 (first English) edition. There, laid before us, is a complex marriage of documentation and mythmaking, image and text. A vivid account of East London poverty, doss houses, park bench sleepers, charity prisons, the grind of labour, spliced with a sequence of uncredited prints that do not illustrate what is written about, but which shadow the story. A kind of interrogation, a challenge to its louder assertions. Some of the images, it appears, were agency photographs, standard archive material; but many were taken, on the hoof, by London himself.

The prose is jaunty, muscular, American, aggressive: London sees horror and wants to do something about it – regime change. Thin, consumptive men, ribs like toast-racks, the shadow of death on them: a different species. But the photographs are much cooler: fixed gaze. They spurn rhetoric. Some, perhaps those for which London was responsible, are like frame-pulls from a travelling shot. A man walking, snooping. Like Robert Frank, much later, stalking the City, fascinated by stockbrokers’ uniforms, the hats. Wandering the East End: sepulchral streets, hearses with open doors, mattresses on waste lots.

The people photographed are unaware, or half-aware, a pause in the business of the day, survival. They are too tired, too engrossed in their own lives, to care about this incident of soul-theft. Being ‘immortalised’ but not named (as you might in a portrait by Julia Margaret Cameron), confirms the modesty of your status. A type and not an individual.

And then: the invisibility of the photographer becomes the invisibility of the subject. Stephen Frears, pitching Dirty Pretty Things, his lowlife romance (TV concept expanded to cinematic proportions), speaks of asylum-seekers as ‘invisible’: an underclass of the unseen (until they are featured on film, the painless way to deal with such things).

In truth, economic migrants are all too visible: in tabloids, in holding camps, rushing the tunnel, hanging about on the coast, talking on cell phones, trainers and leather jackets, not allowed to work for six months after arrival. In limbo.

Some, smuggled in, having endured god-knows-what (we’re waiting for documentarists to tell us), work in kitchens or sweatshops. A tradition that goes back to the pogroms. Whitechapel (which some transients mistook for America) has always welcomed new hands: 15 or 20 to a room in Princelet Street. The march from Irongate steps. Tailoring. Outworkers. We know about this. We’ve seen the colour supplements. We’ve read the ghetto writers, escapees to gentler zones (Golders Green via Hackney). Israel Zangwill makes a picaresque comedy of Jewish life. Emanuel Litvinoff remembers physical details: “Cohen […] in a working apron, fibres of upholstery flock stuck to his gritty chin by a film of sweat. There was something terribly wrong. He looked sick and the pupils of his eyes were dilated in fright.” The hero of Alexander Baron’s The Lowlife is a Hoffmann presser who likes to gamble. And to read.

Anyone exploring the purlieus of London will have heard the noise of machines through open doors on hot days. The smell of chickens, claws in gutters: Brick Lane before its makeover. Railway arches in Hackney, women on pavements waiting to be let in. Vans being loaded. Yellow tin notices: murders, assaults, that have to be printed in three languages.

Jacob Riis, in 1890, called his book, How the Other Half Live, a sensationalist expose. First, we are troubled, sympathetic – and then we feel better. It’s not us. We have moved on, assimilated, changed our names. Riis traps frightened creatures in a dark alley, life under a stone. He presents: Sweatshop in Ludlow St., New York. Pure theatre: too much happening too quickly. Die at your bench and you’ll be replaced in minutes. The machines never stop. Established immigrants employ and exploit newcomers, greeners fresh from the old country. The war zone.

Gautier Deblonde has a very different approach. We have admired his portraits of artists (in their customised sweatshops, studios that will very soon be swallowed up by developers). Artists (performers, performing themselves) are the artisans of the new culture. And how subtly he challenges them! Such silence and steely discretion. Men are often busy or involved with what they have made. Women, immaculately, are responsive to the intrusion of the camera. Rachel Whiteread, time out, on a chair. With lovely balanced shadows.

When he turns (in New York) to sweatshops, Riis’ turf, we know that Deblonde won’t, without invitation, go beyond the permissions of the street. He sees what is to be seen by any interested pedestrian, the city walker who takes time to stop and contemplate a noisy building, hot windows on a cool night. Workers in the dignity of their encounter with a man and his camera. Portraits are taken head-on: articulate passports. Tickets to a richer life: evidence. Workers and buildings. Activists, union representatives. No pathos or special pleading. The small firm architecture of the human presence.

I have some experience of Deblonde in his photojournalist mode. A sticky night walk, by way of Tower Bridge to a now-vanished pub in the shadow of Jack London’s Monster Doss House. I know what it feels like to be, unexpectedly (and painlessly), one of his subjects. He gives as much as he takes. So that your portrait, and the landscape of the bar, the landlady, the other drinkers, is also a portrait of the unseen Gautier Deblonde. A true dialogue. And if any soul has been stolen, it is his own: the perpetrator. Price paid, the slender portfolio will contain a fragment of the living world.