But in the last two centuries, this law has been exercised far beyond the call of common sense. For not only has the museum-going public been told not to touch the objects, it has been encouraged not even to entertain the idea. To this end, those artefacts that seemed to demand some sort of tactile response gradually lost out to those that seemed to offer a purely optical experience. In other words, pictures were given precedence over plastic art.
In the 18th century, it was generally assumed that public museums should show antique sculpture and paintings together. But during the course of the 19th, all sculpture (antique, Renaissance, modern) came to be regarded as inferior to, or less interesting than, painting.
The National Gallery, founded in 1824, consistently resisted efforts to make it exhibit sculpture. The inaugural exhibition in the Sainsbury Wing was consistent with this policy: it was devoted to paintings from the Royal Collection. Ironically, one of the greatest Renaissance masterpieces demonstrated just how much priorities have changed. Lorenzo Lotto's portrait of the Venetian collector Andrea Odoni (1527) is a homage to feel and grip. Odoni is surrounded by antique fragments, notably Hercules wrestling with Antaeus. His left hand lingers on the fur fringe of his robe, and with his right he proffers an Egyptian statuette to the viewer. In the context of an exhibition, and a gallery, entirely given over to paintings, Odoni's hand seemed to symbolise a culture immeasurably more tactile than our own.
The culmination of this two-dimensional prejudice has been the modernist 'white cube'. Here textured wall coverings, dado rails and ornaments were banished; plugs and light-switches were hidden; seating (if there was any) had no arms or backs; individual frames no longer touched, or came close to touching, but were separated by oceans of white or off-white space. Not surprisingly, the white cube coincided with a period in which modern art was regarded as a struggle for a purely optical style - for the 'innocent eye'. It was overwhelmingly a story of painting, which went from Manet to Pollock.
In this rarefied realm, sculpture was all too often regarded as a vulgar, Dadaist irritation. A famous quip by the American abstract painter, Ad Reinhardt, said it all: sculpture is what you bump into when you back up to look at a painting. It was a tactile accident waiting to happen. Even now, the public only gets to touch sculpture by strange default in those museums that exhibit it. If you want 'hands-on' experience (wearing gloves, of course), you need to be disabled, preferably with something seriously wrong with your eyesight. Touch is always the sense of last resort.
But since the Sixties the outlook for three-dimensional art forms has improved. There has been a growing demand for art that is less rarefied and aloof, more palpable and interactive. This has created the climate for a number of successful sculpture movements - Assemblage, Minimalism, Arte Povera, Earthworks, New Object Sculpture, Neo-Geo, Neo-Minimalism etc.
In fact, sculpture has tried to colonise every area of human activity, and to exploit every media and location. Joseph Beuys, for example, decreed that speech and thought are moulded like sculpture, while Gilbert & George claimed to be 'living sculptures'. The other side of the post-Sixties coin is the return of objects and cheek-by-jowl hangs to galleries. Taken together, this suggests that some of the ground lost is at long last being clawed back.
Nowhere are contemporary sculpture's predatory instincts seen more clearly than in the art of installation. Although installations are temporary, in the most radical the sculptor seeks to transform the regulation white cube almost beyond recognition, and thus disorient the viewer. One of the most memorable, ambitious and, indeed, tactile installations of recent years is currently on show at BAC (formerly, Battersea Arts Centre). It has taken the 34-year- old British sculptor Andrew Sabin over a year and pounds 40,000 to concoct. Though its Turner-esque title, 'The Sea of Sun', would appear to situate the installation within a pictorial tradition, Sabin's pictorialism is powerfully three-dimensional, and only comes to life through touch.
The gallery consists of two large adjoining spaces, connected by a small central door. Sabin has sub-divided these spaces into 26 square and rectangular compartments by suspending screens, made from 28 kilometres of aluminium chain, from a false ceiling. The links have been anodised in a variety of colours and assembled to make glossy abstract and figurative tapestries. Because the screens are semi-transparent, the images bleed into each other psychedelically. Moreover, when the viewer walks through them, they sway and clink.
In the first room, the imagery is primarily abstract, luridly coloured blobs and squiggles. It implies a primordial, preconscious state. Passing through these screens is like plunging into a sci-fi jungle filled with giant spores, microbes and bacteria. In at least one of these biomorphs, it is possible to imagine the squid-like lineaments of a face by Munch. It could almost be a Disneyland for the AIDS and Ecstasy era.
Sabin says that he wants his work to function like virtual reality, and to give the viewer 'as little chance of avoiding sensations as would be given by a sudden shower at a bus stop'. But if this is a shower, it is one that feels cold and looks acidic. One suspects that he wants his viewers to be squirming, as well as singing in the rain.
The imagery starts to coalesce - or mature - into recognisable shapes near the door into the next room. There is a landscape with trees, a screen inscribed with the word GREEN, and a black-and- white silhouette of a Renaissance arcade. These are a bit too vague and generic to really get under your skin, but the arcade could be intended to conjure up a memory of Venice - that floating world of illusion and intoxication.
In the second room, this kind of imagery is supplanted by black-and-white head shots that show three generations of Sabin's own family. These occupy entire screens and hover in front of you like great dictators. The way they hang there, their vertical tracery shimmering in the bright light, recalls Bacon's use of 'shuttered form' to modulate the surface of his paintings.
Yet the aggression of these images is matched by the aggression that gets meted out as we pass through them, or flick the faces with our hands. When the chains ripple and heave, the images are shredded as though they had just had a close encounter with Norman Bates (ah] that shower . . .) or Edward Scissorhands. The black comedy does not stop there. After a momentary deformation, the chains stop swinging, and the wraith- like images reform. The thrust of Sabin's work is especially obvious shortly after Christmas: nothing can make our closest relations disappear from view or from consciousness for long.
You can see why a museum director might have qualms about admitting such tactile temptations into the heart of his empire. A dangerous precedent might be set if the public got confused and started to think that the Rembrandts could be stroked or shredded with impunity. In positing that the public can take matters into their own hands, Sabin has touched a raw nerve.
The urbane Spanish surrealist Joan Brossa, in an exquisite retrospective at the Riverside Studios, suggests a solution to the problem of an uppity audience: shoot them. 'Interval' consists of three wooden chairs, three stands for sheet-music, and, propped on the chairs, three machine guns. Because, however, these items have been placed at our level, rather than on a podium, we cannot be sure whether it is the absent performers or the audience being invited to pull the trigger. In the end, the triggers are too close for anyone's comfort.
Another intriguing masquerade takes place with 'Yacht'. This consists of a bath, again placed directly on the gallery floor, which has been filled with unframed paintings of seascapes. Though these canvases stick up like sails, the bath/yacht is metaphorically filled with water. Evidently, if you put painting and sculpture together it is a recipe for disaster and farce. The paintings have been taken from their wall and from their frames in order for the yacht-sculpture to exist, but at the same time, they wreck it, because they fill it with water . . .
The launch of the 'un-Television Video Gallery' suggests that the eye may yet have the last laugh. U-TVG is a quarterly video arts magazine that aims to document exhibitions, interview artists and create a platform for artists to make or show their own films. The first edition covers Sabin's installation in depth.
U-TVG brings art right into the home, but there is a danger that it will allow us to see art at a safe, optical remove. It is an excellent idea, but it would be even better if each video came with a small sample of any touchable material - for starters, how about a few links of Sabin's chain?
Andrew Sabin, BAC, Lavender Hill SW11 until 15 Jan; Joan Brossa at Riverside Studios, Crisp Rd W6 until 10 Jan.
For further details of U-TVG, write to: PO Box 104, London SW19 5UP.