Great Works: The Vision of St John, 1608-14, By El Greco

Metropolitan Museum of Art, New York

Paintings can remain unfinished for many different reasons. This fact can turn them into enigmas. The possible causes of their abandonment can be so various: death may have intervened or something as prosaic as a more pressing commission; the artist may have hit a creative block of one kind or another; a patron may have cooled.

In the case of many of the late canvases of JMW Turner, we simply do not know whether the present condition of these works was intentional or not; he often didn't tell us. What we do know was that he was quite regularly in the habit of resolving a painting at an almighty rush, over a single feverish working day. His admirers (and cowed detractors) could then witness the awe-inspiring spectacle of the master wresting something from nothing – like God busily at work on his creation.

In the 17th century, it was not unusual for an artist to complete a painting in part, and then to wait for a likely purchaser to express an interest in it before completing it – perhaps at an indecent speed. We may never discover why this painting by El Greco is probably incomplete. What interests us just as much is its afterlife.

So much that happens is accidental. At the turn of the 20th century El Greco's reputation was at last on the rise, having been moribund for centuries. Why take seriously an eccentric mediocrity? Little by little, opinions changed. Degas saw something very special in him, as did Rainer Maria Rilke, the great poet and sometime secretary to Auguste Rodin. He seemed to be one of us after all. The intensity of El Greco's colours, his oddly elongated forms, that wild, visionary gleam in his eye – all these factors helped to turn him into a precursor of modernity.

Pablo Picasso saw this monumental altarpiece, commissioned for a church just outside Toledo, in the studio of a Spanish painter friend in 1906, and it quickened something inside him. It also kindled an urge to express the idea of the Spanish soul. What is more, and more specifically, it had a profound influence upon the painting that became known as Les Demoiselles d'Avignon. What Picasso might not have guessed is that the fact this altarpiece is probably unfinished invested it with qualities that it might not have possessed had it been polished to a higher level of perfection.

The subject is taken from The Revelation of St John, the final book of the New Testament. The evangelist himself, standing on the extreme left, seems to rise out of his own shimmeringly columnar body, as if aspiring skyward. His fingers seem to be pushing the paint around as if to assist in the artist's wildly turbulent execution of the sky. Huge bolts of cloth, green, yellow and pink, seem to writhe and ripple, as if alive, as if they aspire to that condition of protean shapelessness possessed by water. Seven naked figures claw at and reach out towards this cloth. What exactly is it though, and what purpose does it serve? The text describes how robes are given to the souls of those who have been killed for the word of God. You could therefore call this altarpiece a species of consolation to the faithful.

The moderns would have admired the fact that there is something very rough and ready about this painting. It is all no-holds-barred, rushing, provisional gesture, something caught on the wing. There is no heightened polish here, no solid, utterly dependable, old-masterish, browny dullness of the kind that Joshua Reynolds so strove to emulate with his treacly, undryable bitumen. The spaces are ill-defined, distances are fantastical, issues of perspective have been blown to the four winds.

It is the compelling atmosphere of this work that seizes us by the scruff though. This painting looks molten to the touch, as if it is still being made. And this is precisely what we too, latter-day moderns all, like so much about it. We want paintings to look provisional – think of the canvases of Peter Doig, finished enough but never too much. We want to be reminded that paintings are made things. Old-fashioned illusionism feels too pat, a species of fakery. So there is much flatness – or near flatness here. These gesturing, naked bodies, those sky-turning, sky-tumbling, backwards-somersaulting putti-like angels – surely they are no more than three quarters done. Entire mouths, hands, chins are smoothed away – or are left unpainted. It is as if we are seeing them through a fudgy mist of intense light, the light of the visionary moment, just before everything becomes clear. We like this feeling of being on the cusp of revelation amidst bodies that seem to be moving towards their full identities, their full shapeliness.

What is more, the whole thing was brutally trimmed down during its "restoration" in 1880. A whole section of the upper sky was removed. "Good," we think to ourselves. We quite like how that sky seems to be closing in on the squirming fingers of St John.

About the artist: Domenikos Theotokopoulos (1541-1614)

Domenikos Theotokopoulos, nicknamed "The Greek" and born on the island of Crete, trained as a painter of Byzantine icons. Before the age of 20 he moved to Venice, where he trained under Titian, and then later to Rome. He spent the greater part of the rest of his life in Toledo, where he painted many of his most important paintings. Regarded as a mediocrity until the beginning of the 20th century, he was dramatically reclaimed by the moderns as a precursor of everything they were striving to achieve.