The Hayward Gallery's ''The Art of Ancient Mexico'' quickly establishes itself as one of the better hung exhibitions in London at the moment. Six fertility goddesses on tall plinths preside sternly over the opening gallery, which also includes a 3ft-long stone phallus from the Museo Nacional de Antropologia in Mexico City. This daunting object was discovered a century ago in the plaza of a small Mexican town called Yahualica, where it played its part in an archaic local custom said to have involved a lot of flowers and a complicated fertility dance.
The installation at the Hayward discourages ritual dancing as a response. Labelled exhibit 81, the spotlit phallus is perched on a high ledge beside a snarling Olmec Humanised Jaguar and other relics of ancient Mexican votive cults. It is an unlikely survivor of the Spanish conquest. The evangelists who set out to convert the Mexicans to Christianity did not look too kindly on penis-worship, so most of these stone phalluses were broken up and replaced by Catholic cult objects: statues of the Virgin and the like. Talk about going from one extreme to the other.
But preserving things can itself be a subtle way of destroying them, or at least of denaturing them. Confiscated from the people of Yahualica by 19th-century anthropologists, placed in a museum and now on loan to the Hayward Gallery, Phallic Sculpture exists, these days, to be admired as art. One form of reverence has turned into another. Quite what this transformation has done to the object remains arguable. It makes the Hayward show fertile ground for debate.
To see an exhibition of ancient Mexican sculptures in a modern art gallery is, it might be said, to see them doubly distorted. For one thing, it is to be tempted to see them as art in the modern sense, as objects of intellectual or aesthetic contemplation divorced from their older ritual contexts. And it is also to be tempted to see them not just as art but as modern art before the fact. Their most pronounced formal characteristics - their harsh, angular geometry or their distortions of the human anatomy - can easily make them seem prophetic of the innovations of early modernism. Phallic Sculpture is easily thought of as a prototype for Brancusi's similarly phallic-abstract Princess X; the diagrammatic face of the Western Mexican Mask from Mezcala as a pre-Modigliani Modigliani; and the intimidating figure of Chac Mool, a glaring vengeful Mexican god of sun and fire, as (with a little creative imagination) a Henry Moore reclining figure.
The visual similarities between ancient Mexican art and modern sculpture are not coincidental. To see a resemblance between a Huastec monument and a Brancusi is to find what Brancusi put there. Modern artists preoccupied with the ''primitive'' spent a lot of time in the ethnographic departments of institutions like the British Museum and deliberately built references to Mexican art, among other things, into their own work. But the legacy of their appropriation of the devices of ancient art has been to alter, perhaps for ever, modern perceptions of it. We see it, not with innocent curiosity, but through the distorting lens of reinterpretation and reinvention.
The modernist approach to the art of ancient Mexico was tinged with sentimentality (''Primitive art is something made by people with a direct and immediate response to life,'' said Moore) and coloured by all sorts of other anachronistic ideas of what its original creators might have been trying to express. One consequence of this, easily detected by eavesdropping on visitors to the Hayward show, is a tendency to admire such art primarily for qualities like immediacy, or formal daring: ''Look at how wonderfully stylised that figure is - just like an Epstein''; ''What a wonderful head - it's so abstract, just a few lines cut into a sphere''.
But this sort of remark represents a wonderful misattribution of ''civilised'' modern aesthetic preoccupations - with form or mass, or the development of an abstract vocabulary - to people who were really not very civilised at all, at least by the standards of the average Hayward visitor. After all, a 10th-century Mayan's idea of cultural self-expression might well have involved the odd human sacrifice or two, and the sculpture of Chac Mool that inspired Henry Moore is rather harder to admire for its purely formal attributes when you know that the dish which the figure holds was once the repository for human hearts ripped from the chests of sacrificial victims. Likewise, it seems oddly beside the point to admire the proto-Cubist distortions of a Teotihuacan Ceremonial Sculpture in the Form of a Skull when you find out that its blanched, worn surface of bare stone might well have been originally designed with a liberal coat of human blood in mind.
None of which is meant to suggest that it is wrong, exactly, to refer to ancient Mexican art as art, but it is worth bearing in mind that the people who made these objects did not have that much in common with your average modern artist. There is a difference between ''I've got to get this sculpture finished for next week's RA Summer Show'' and ''I've got to get this sculpture finished for next week's ritual murder of captives.''
In fact, the Hayward exhibition deals extremely skilfully with the difficult problem of how to present the art of a largely mysterious, geographically and temporally distant network of societies. It is a show with a conscience. It is prepared to question the very assumptions which, as an exhibition of ancient art staged in a modern art gallery, it could have been merely content to confirm.
Paul Williams's fine installation design is largely responsible for this, because it predicates the whole show on the notion of distance, which becomes a metaphor for the immense cultural void that separates these objects from those inspecting them. Votive statues are placed on high, high plinths; sculptures are placed behind glass; or, where they are closer and more accessible, the lighting works to enhance a sense of their forbidding separateness, placing them in spotlit isolation. You are never allowed to forget the sheer alienness of what is being contemplated.
Stonily silent, aloof, Chicomecoatl, Huastec goddess of maize, closes her eyes and tacitly reproves anyone with the temerity to see her as just a work of art. She remains withdrawn: physically present in our world for a while, perhaps, but spiritually absent. The exhibition contradicts its own nature as an art exhibition by metamorphosing into something else: a gathering of strange deities, a conference of forgotten, once-potent gods.
It is easy but also lazy and perhaps even vaguely immoral to see these objects through the lens of modernism - because such an approach demands no adjustment of our habitual ways of seeing, no work, no attempt to imagine the nature of the cultures which produced them. But it would probably be nave to suppose that we can ever truly grasp what the curious, distorted forms of these sculptures signified to those who created and worshipped them, or decipher the impassive, enigmatic expressions of these gods and goddesses, these hybrids of man and animal. Too little is known about the complex beliefs of the Huastecs, Toltecs and other ancient Mexicans. But it seems certain enough (for example) that the schematised, abstracted likenesses of woman produced by the Huastecs, where the emphasis falls so heavily on the belly and breasts, have more to do with notions of fertility than with some proto-modern conception of expressive distortion.
''The Art of Ancient Mexico'' challenges just about every modern preconception about the art of much older, alien societies. Ancient Mexican art was not modern art waiting to happen. Ancient Mexican art was not any more immediate, any less governed by convention, than the art of the modern Western tradition. Henry Moore was wrong to think it was ''made by people with a direct and immediate response to life'', since it is evident that the art of the ancient Mexicans does not present life in some innocently direct manner but, rather, mediated through a highly sophisticated system of conventions: its forms amount to highly stylised reinventions of the appearance of things, a complex set of visual codes for people or animals evolved over centuries in response to the pressure of religious beliefs and superstitions.
The Hayward's exhibition also dents the currently fashionable, politically correct conception of the primitive artist. It reminds us that, in our terms, these people were not ''alternatively culturally oriented''. They were barbarians who offered up human sacrifices to savage gods of stone. Sophisticated, skilled, complex barbarians, perhaps - but barbarians never the less. This exhibition seeks to restore, to primitive art, its most important attribute: primitiveness.