The Naked and the Nude, Middlesbrough Institute of Modern Art, Middlesbrough

Why are some subjects of paintings described as naked and some as nude? The old debate rages on in a revealing new exhibition

The title of Philip Wilson Steer's painting, Seated Nude: The Black Hat, raises an interesting question, namely whether you can really be unclothed if you have a stack of feathers on your head. Nudity is a slippery term, as Steer's picture suggests. A century after it was painted, his work has the power to embarrass. The truth is that Steer's nude isn't: she is naked. The difference between the two is the subject of the opening chapter of Sir Kenneth Clark's book The Nude, now the inspiration for a show of 20th-century British paintings at the Middlesbrough Institute of Modern Art.

Hung in rough chronology, The Naked and the Nude begins with Steer's after-Degas lady of circa 1900 and ends with Euan Uglow's Zagi, painted in 1981-2. Although, thanks to the linear nature of the hang, these have ended up next to each other, the two works could not be less alike. The titular black hat of Steer's canvas hides its wearer's eyes. This has the dual effect of making her guilty – the hat is the Edwardian equivalent of the pornographic black bar – and of blinding her. We can look at Steer's model, but she can't look back at us. In this, she is the precise opposite of Manet's Olympia, who meets our gaze with an unblinking gaze of her own.

Contrariwise, Uglow's subject is nude rather than naked. Facing left to right and standing at the barre, her resolute disengagement from the world seems inhuman. And it is: Uglow's model was not a woman but a Chinese anatomical doll. The obsessively exact modelling of her surface – "flesh" is hardly the word – takes on ideas of the naked and the nude only to dismiss them. Uglow might as well be – is – painting plastic. As the grid marks on Zagi's surface show, he worked very slowly, measuring pigments by weight and lining up perspective with string. The work's real subject is neither nakedness nor nudity, but the perfection of Uglow's method.

Two depictions of the same subject with contrary aims: how come? Clark's answer was that nakedness "implies some of the embarrassment which most of us feel in that position", while nudity shows "the body re-formed". The writer and critic John Berger saw things the other way around. "To be naked is to be oneself," he said. "To be nude is to be seen naked by others."

This shift in view is arguably historical, Clark having written just before the 1960s, Berger just after them. But to see a parallel shift between Steer and Uglow would be wrong. Steer's isn't simply an Edwardian painting and Uglow's a work of the 1980s; they sit at opposite ends of a scale of possibilities explored by Titian or Ingres or Picasso. Clark and Berger were both right – the most erotic painting of an unclothed figure is still a nude, the most academic a study in nakedness. Whether you're a Bergeristor a Clarkean, the shadow of the other will always be there. It is this duplicity that makes the bare human form such a compelling subject for representation, as the works at Mima, lent by the Tate, variously show. To Lucian Freud, his wife's single, naked breast in Girl with a White Dog is both intimate and voyeuristic; Freud uses the gap between these two things to explore the possibilities of modelling and colour. David Bomberg's Lilian is so abstracted as to dress its nude subject in a kind of clothing; it is, even so, a portrait of his wife with no clothes on.

The only work in this show with no tension between nakedness and nudity was painted in 1922 and is called, simply, Nude. It is also the only painting by a woman, Vanessa Bell. There are no pictures of naked men in this exhibition, let alone pictures of naked men by women. There is a book on the subject in Mima's reading list for The Naked and the Nude, but it is locked away in the library and – make of this what you will – you need special permission to see it.

Mima, Middlesbrough (01642 726720) to 16 Nov