There's a picture by the Renaissance Venetian artist Giorgione called The Tempest. It's famous for being baffling. It shows a landscape and a town, a flash of lightning in the sky, two isolated pillars, a soldier, a naked woman and a baby – an all too meaningful ensemble. But what on earth does it mean? A story? An allegory? Art historians have wondered, and no answer has been found.
It's hard to know even what the problem is. Perhaps the meaning was perfectly clear once, but now it eludes us. Perhaps there was never a meaning, only a dreamy poetical scene. And there are other possibles. It could be a puzzle-picture, with clues to be deciphered. It could be a trick, a pseudo-puzzle with no solution, designed to waste your time. It could be a conscious tease, which asks to be enjoyed precisely for its brilliantly constructed mysteriousness.
Though remote in many ways, The Tempest reminds me of Ed Ruscha's art. This West Coast painter is a master of mystery. To use that notorious phrase, his work is full of unknown unknowns. Take his picture from 1998 entitled The Mountain. It shows, unsurprisingly, a rising snowy peak. But then, superimposed on this view, is a word, three letters, upper case: the. The?
And at once, you're puzzling. You might say, well here are two ways of saying the same thing: one verbal, as in the title, one verbal-plus-visual, as in the painting. The. Mountain. But the way the letters are superimposed makes the the look more like a definition of the mountain, or as if the mountain was the very essence of The-ness. And see how the t on the the is much bigger than the other letters. Something deeply odd is going on between word and image. What does it all mean?
Ruscha – incidentally, you pronounce it Roo-shay – is in his early seventies. About 50 years of his work can be seen at the Hayward Gallery, 50 years of carefully composed enigmas. It begins with words.
Around 1960, many American painters were looking for a way out of pure abstraction. Ruscha took the way of letters. He painted words, often single words, drawing inspiration specifically from public lettering – from shop fronts and signboards, and product advertising with its trademark type-faces. There are pictures with bold and punchy logos: war surplus, boss or oof. There are pictures based on the 20th Century Fox emblem and the Hollywood sign on the Hollywood Hills.
In these word paintings, words don't perform as they normally do. They don't communicate. They go dumb. It's like that odd speech phenomenon, technically called "semantic satiation" or "semantic saturation", which happens when a word or phrase is repeated so much that it loses its meaning, and becomes just an empty sound. That's what Ruscha's early paintings do to the printed word. Their letters become semantically null, just a formation of cut-out coloured shapes.
Or rather (and this is the tantalizing thing) the words don't become totally abstract. They don't forget their sense entirely. Rather, they do and they don't. This is why "semantic satiation" – printed or spoken – feels so weird. It hovers between the meaningful and the unmeaningful. And that is an ambiguity you find throughout Ruscha's work.
But the next two decades were not so productive. It's as if he'd got his essential idea, but hadn't found ways to bring it out. Ruscha played games with words, some of them silly (the word boss again, but with the letters squeezed by vices), some very laconic (the phrase sand in the vaseline painted gold in grey). He introduced images too, and found a way to make them feel semi-meaningless, as he had with words.
The trick with images is to paint some object (pills, a building) in a rather precise and illustrational manner – but to set it, in isolation, against a blurry and almost blank background. It puts the object in quotations. You see it as an opaque piece of painting. You don't see through it, to the thing that it depicts. Or rather, again, you do and you don't.
Then, in the early 1980s, Ruscha devised what became his signature device: words floating over a view. You might say this device was borrowed from magazine ads or book covers or title sequences of films, except the paintings don't look much like any of these. Their words often use Ruscha's personal typeface, a clunky, curve-free font (the O is an octagon). And the relationships between the words and the views behind – well – they're absolutely gnomic.
not a bad world is it? floats over a kitschy sunset landscape, that looks like a vision of heaven from a Jehovah Witness leaflet. Meanwhile, over a cityscape at night, with its grid of lit streets, talk radio is sharply written in neon. And then american tool supply is superimposed on a picture postcard snowy mountain range. Or sometimes it's not words but an object that takes the foreground. Five Past Eleven shows a very large close-up section of an old Roman-numeral clock-face; floating over it, a very fine length of bamboo cane.
What connects these juxtapositions? Irony, surrealism, specialist history, a private memory? Who knows? And Ruscha has other tricks of obscurity. His phrases are plucked from some unimaginable context, like the charming faster than a speeding beanstalk. Figures are shown in such blurred silhouette that they almost retreat into invisibility (see the magnificent coyote in Howl), or likewise in a dazzling glare. Cartoon caption boxes appear, but with no words in them. They all go into the mix.
You might think of the game of word-non-association. Each player has to say a word in turn, and it must have nothing to do with the word said by the last. It's strangely difficult. Words have their gravitation. Ruscha is like an expert player of this game – but with this refinement, that each time he seems to have fallen into an association, yet when you try to work out where the association is, you can't.
That's how his pictures work. They're painted so clearly, and they seem to add up so clearly, that you smile. It's obvious. But when you try to put your finger on it, the meaning eludes you, just, but completely. I'd especially recommend (though I couldn't get it across) the picture called Oldsmobile. There's a meaning there, surely, but utterly out of your grasp.
And along with this elusiveness, and stranger still, there are open hints of the transcendental. True, it's not Ruscha's only subject. His images and words refer to industry, city life, the big highway. But there's enough to make you wonder. There's the ghostliness of his blurs, the blaze of illuminations. There are the soaring mountains and the heavenly clouds. There's the explicit religiosity in his words: 90% angel 10% devil, a particular kind of heaven, hell heaven, sin. Perhaps the strangest image here is words without thoughts never to heaven go, arranged in a neon ring against darkness, the words diminishing as they go round. It's from Hamlet, Claudius trying hopelessly to pray.
So the final enigma of Ruscha's enigma variations is where their mystery stands. Is this art, with all its ungraspable beyonds, a kind of religious art? It presents us with appearances and signs. It asks whether there's something or nothing behind its words and its images. His hieroglyphic images constantly provoke and defeat our attempts at interpretation. Are they blank, empty facades? Or do they imply sublime inarticulable depths? Don't worry, you'll never know.
Ed Ruscha: 50 Years of Painting, Hayward Gallery, London SE1 (0871 663 2500; www.southbankcentre.co.uk) tomorrow to 10 January