Goya's gruesome themes still resonate today

Rediscovered treasure... Francisco Goya's The constable Lampinos stitched inside a dead horse. Photograph: Christie's/PA

The world has three new masterpieces - three shocking, grisly, comic, brutal revelations of human folly from the hand of Francisco Goya.

They come from the period in the 1790s when the Spanish court painter was digging deep into his imagination to dredge up the scenes of madness and mayhem that place his late works among the bleakest and most honest in the history of art. And they speak to us today, directly.

The rediscovered drawings instantly join the ranks of Goya's great works. One depicts women falling through space, their faces contorted in fear or exhilaration, while another takes a grim view of Catholic piety, with a man torturing himself in his "repentance". In the most gruesome and brilliant of all, a man's head, howling in misery, pokes out of a horse's arse. He's like a living version of the mythic centaurs of ancient Greece: in myth, and in paintings like Botticelli's Pallas and the Centaur, this half-man, half-horse monster has a muscular human head and chest emerging elegantly from a sleek equine body.

Goya's tragicomic "real" centaur resembles a cruel fairground attraction. His head, a round, dumb, provincial head, emerges wretchedly from the horse's rump. With his blob of black hair, he's like a turd. Then you realise the horse is dead: a row of dark lines on its stomach show where the eviscerated carcass has been stitched back together. This poor man is suffering some grotesque punishment, some monstrous carnival joke.

Goya's written note on the sheet relates the story his drawing illustrates. It is a "true story". In 18th century Saragossa, the people took revenge on an oppressive official called Lampinos by sewing him inside a dead horse. He survived this torture for an entire night. The image is a terrific, and terrible, addition to Goya's visual catalogue of the follies of Spanish popular culture.

In his painting The Burial of the Sardine, on display in the Royal Academy of San Fernando, Madrid, masked dancers swirl manically below a banner of a stupid, grinning face; the crowd's lumpen delight is scarily irrational. Pilgrimage to the pool of San Isidro, among his Black Paintings in the Prado, portrays another maddened crowd.

So, why is Goya so popular today? Here are two answers. One is his newness: by which I mean his apparently limitless and virtually unrivalled ability to generate form. He doesn't so much reject as completely ignore the classical lexicon of poses that still informed most art in his day. His bizarre formal compositions and physiognomies seem to materialise out of nowhere - or out of the Spanish reality in front of his eyes - and this brings us to the second reason for his authority today.

Goya strikes us as true. His most macabre fantasies don't seem merely playful or surreal but actually capture the reality of what people do, every day, to one another and to themselves. He is the journalist of unreason: a man of the Enlightenment who hoped for liberal reform - in his portraits he pays homage to enlightened contemporaries such as the government minister Gaspar Melchor de Jovellanos - but who came to doubt if it was possible. His art abounds in satires on religion and conservatism and yet, for every depiction of the barbarity of the Inquisition, there is another, like his "new" drawing of a man sewn by the people in a horse's stinking carcass, that records the brutality of mass action.

I think that Goya is our conscience. He lived in an age of spiralling unreason, and so do we. Goya ended up seeing no sign of true enlightenment in any of the movements of his time, as should anyone who is honest and thoughtful today. Goya's painting The Third of May 1808, in the Prado, often reproduced and interpreted as an indictment of state power, is actually part of a pair. The painting that goes with it, The Second of May 1808, shows the rebellion that prompted the next day's executions as a desperate, deranged, cruel affair. The rebels attacking horses with daggers are not paragons of reason and liberty but manic killers.

When photographs of the torture of Iraqis by American soldiers in Abu Ghraib prison emerged, it was disturbing and frightening to see how uncannily the guards' callous games echoed scenes from Goya's art. And yet you can equally well see analogies between his depictions of Catholic Spain and today's religious terror.

Goya takes no sides. He saw our black time coming. Today, people argue routinely that 9/11 was an American or Israeli plot, that evolution is a myth, that immigrants threaten instead of enriching societies, that medievalist political groups who kill at random may be punishing those who had it coming... and so it goes. As Goya inscribed on one of his most famous images: The sleep of reason produces monsters.