The wurst of the 20th century

Sigmar Polke is one of the most brilliant and important artists working anywhere in the world today. Since 1990, a series of revelatory retrospectives in America and Europe has enthralled public, artists and critics alike. At last, at the Liverpool Tate we can see for ourselves what the fuss has been about. I cannot say how important it is for you to see this show, but see it you must, and be changed. It establishes without doubt to my mind that Polke possesses the greatness that has been claimedfor him, and that what his work has to tell us is necessary and urgent. How it tells it is another compelling reason to go to Liverpool. Polke is an artist, not a politician or a moralist or a theorist. He has not forgotten that art by definition works aesthetically: it enters the mind through the eye. If affects our thoughtand feeling through the immediacy of its impact upon the senses, and through subtleties and indirections in the information it imparts. There are some stunningly beautiful paintings in this show, and there is a largeness of spirit in them that can inscribe the spectacular with irony and throw a poetic light in the commonplace. Polke finds material equivalences for profound ideas in cheap fabrics and corrosive substances, and invests banal objects - potatoes, tea towels, doughnuts, press photographs - with magical changes of meaning.

He knows the power of a joke in the darkest of contexts. In Paganini, the ambitious but flawed masterwork of 1981-83, the composer-virtuoso - the very figure of individualistic genius - lies dying while a Nazi devil at the bed's end has stolen his fiddleto conjure up a diabolical Romantic sturm. This sweeps across a patterned-fabric ground suggesting bare wartime fields (we have reaped the whirlwind) whose zigzag divisions are foreshortened swastikas, and ends in a cadenza of skulls turning into nuclear emblems. At the dead centre of this demonic chaos the handle of Paganini's bedside cabinet is the shape of a perfect sausage.

Polke draws on the great charge of anarchic alternative energy that flows through Modernism from Dada through Schwitters, Hannah Hoch and the Arps to Rauschenberg, Klein (just about to open at the Hayward), Warhol and Beuys (his mentor and friend). It isa line of force that confuses the closed categories of art history and the hand-me-down cliches of art journalism, sparking connections across stylistic, historical-political and geographical divisions. Out of it emerges the image of the artist as explorer and scientist, whose personal journey of discovery is exemplary, not a matter of individual genius. Like his predecessors in this dynamic line, Polke renounces signature and personal style, making his art of whatever the world presents.

From the beginning he has used printed fabrics instead of canvas, delighting in the opportunities that they offer for pictorial irony or the visual pun. Rejecting the "authenticity of touch" and the expressionistic gesture, his paintings exploit impersonal chance and process, splashing, pouring, staining, corroding. It is an art of appropriations and contradictions, gathering its materials in encounters with the facts and fictions of the everyday world. It borrows images freely from old engravings, photography, newspapers, and gives them back to us transmogrified by magnification and dislocation, blurred, truncated, saturated in colour or bleached, tricked up with arbitrary and unsettling decorative effects.

Much has been made of Polke's apparent will to subvert the traditional techniques of painting and conventions of picture making. He has been criticised for his recourse to disguise and ambiguity, his resistance to interpretation or downright unintelligibility. These faults are seen as implicit in his technical procedures, with their maskings and disfigurations, his use of corrosive and unstable chemicals and unpredictable media. This view of Polke as negatively disposed towards painting has extended to his recent use of transparent lacquered fabric supports, in which the struts of the wooden stretchers are visible elements of presentation of image. But all this is to ignore the evidence of the senses, the sheer exhilaration of standing in a room full of Polke's recent paintings, with their atmospheric incandescences and translucencies, their cosmic brilliance and strangeness, the complex richness of their colours and forms. It seems to me beyond question that he has brought an amazing range of new effects into painting which can serve only to enlarge the scope of its imaginings and broaden the range of its expressive possibilities.

To some observers, Polke's approach to subject matter has also seemed obscurantist, evasive or incoherent. He presents his findings often in allusive and paradoxical images, in which the relations of disparate elements will not easily submit to a rational analysis or be reduced to narrative or commentary. Why is this young knight, trapped for ever in the universe of art as surely as he is imprisoned in a bare cell and entrammeled in his armour, warming his civilian boot? Or is he preparing to burn it? What is announced by the modern dress angel in the iridescent air of Annunciation? We will learn to live with these mysteries without irritability.

At the same time I find no such mystery in the Refugee Camp (1994), only a direct and angry despair at the poisonous air of displacement and incarceration. And the cancerous destruction of the baroque facade in Ruin (1994) perfectly images the degeneration of Romantic enlightenment. Earlier paintings lack this grim sort of grandeur, their politics working rather by a sly ironic charm: in Berliner (1965), a baker looks out over a barrier of doughnuts (berliner is the German name for that confection), recalling Kennedy's famous and fatuous pronouncement of two years before: in the laconic Solutions V (1967) not one of a column of simple sums adds up; and if, in The Higher Powers Command: Paint the Right Hand Corner Black (1969), we read merely the dogmatic prescription of formalist painting, we shall miss deeper references to Cold War art politics, and to the higher orders that resumed command in the year after 1968.

Polke confronts the terrible actualities of the century with wit and grace, and is not afraid of beauty and mystery. His work confuses critics because it lacks the programme and the legibility of intention that can be turned to portentous table talk or harnessed to theory. There is no rhetoric of history or prophecy in these works, but there is memory and warning, a profound awareness of the past, and of the present plight of the planet as a millennium ends. This is an astonishing and marvellous exhibition.

n At the Tate Gallery Liverpool (0151-709 3223), until 21 April n Art listings are on page 19