ARTS: WHAT I'VE BEEN SEARCHING FOR

WHEN THE American photographer Diane Arbus died in 1971, at the early age of 48, she left one especially precious set of pictures. It contained photographs that she had been taking in the previous two years at homes for the mentally retarded. Some of the most telling of these prints have not previously been published. Looking today at the pictures, now collected in book form, we wonder at the nature of Arbus's feelings - and then we wonder at our own, for these strange photographs are not about time and place, but concern the mysteries of humanity.

"Finally what I've been searching for," Arbus wrote when the prints emerged from her darkroom. She did not say much more about them, yet her allusion to finality must be significant. The photographs not only extend but complete her short, remarkable artistic career. A native New Yorker, Arbus studied under Lisette Model - a vital link both with European camerawork and pre- war American documentary styles - in the late 1950s. Her first published photographs ("The Vertical Journey") appeared in Esquire in 1960. She then worked as a freelance for such magazines as Harper's Bazaar, Glamour and Show, while also teaching photography at the Rhode Island School of Design and at the Cooper Union.

Therefore she had a wide experience of her craft. Although she is famous as the photographer of freaks and social misfits, Arbus was also concerned with fashion, portraiture and street photography. Certainly dwarfs, transvestites and twisted adolescents were prominent in her work, and she must have sought them out rather than encountering them by chance. But for what reason did she seek out people who were monstrously disadvantaged? Not, we now understand, because these subjects were so striking and half-forbidden. She approached them because her ambition was to become a photographer of the human condition.

That is the subject of these pictures, and probably the reason why Arbus could not bring herself to title them. Her previous works were known by flat descriptions like "A Jewish Giant at Home with His Parents in the Bronx" or "Loser at a Diaper Derby". Her last pictures, though, might have been diminished by added words. For once, the word "untitled" seems both exact and eloquent. In the last two years of her life Arbus's photography went beyond documentary, the mode that gives us locations and facts. And her subjects are themselves "untitled". They have no property, no home, no hold on the world, only the experience of their own existence. It almost seems they have no names.

Arbus visited and revisited the "residences" she had discovered every couple of months. One guesses that she often made herself a guest at times of minor institutional celebrations. At the residences there were occasional dances. In summertime there were picnics. The autumn was marked by Thanksgiving and Hallowe'en - hence, no doubt, the masks that the patients wear. The meaning of any festival was surely beyond the intellectual range of the people we see here. They still dress up and play. They know that something special is going on, that this day or that day is not like other days. And, perhaps above all, they pose.

Or rather, give themselves to being seen. Their subnormality absolves them from the normal adult's instinctive responses to another person's gaze. Arbus's subjects have a crazy unaffectedness. This makes the observations of her camera distant, but not dispassionate. The camera exhibits a kind of awe that beings on earth could have any kind of social life. This is not an original vision. We have seen it before, in Goya. The Spanish painter must have been an influence on Arbus's last photographs. Where else do we find images that so haunt us and unsettle our notions of rational civilisation?

! 'Diane Arbus: Untitled' (Thames & Hudson, pounds 36) is published on 25 Sept.