When Italian Baroque artist Luca Giordano declared Las Meninas “the theology of painting” in 1692, he was paying an extravagant compliment, “by which he meant to convey,” as Giordano’s friend and Velázquez’s biographer Antonio Palomino put it, “that just as theology is superior to all other branches of knowledge, so is this picture the greatest example of painting.” But just as a passage from Pierre Menard’s Don Quixote is, according to Jorge Luis Borges, “almost infinitely richer” than the original by Cervantes, so this phrase becomes infinitely richer if we imagine it being pronounced by a later admirer like Édouard Manet, who certainly did consider Velázquez the greatest of all painters. A visit to the exhibition of the Spanish master’s work at the National Gallery in London, on view through January 21, makes it hard to disagree with Manet, even despite a big handicap: Of necessity, the show lacks many of the artist’s most celebrated works–including Las Meninas. Most remain at home in Madrid, as the great portrait of Pope Innocent X does in Rome. Still, the Prado, along with other institutions and a few very fortunate private collections, has been generous with its loans, and since London already domiciles the largest concentration of Velázquez’s works outside Spain, the National Gallery has been able to round up forty-six paintings, about a third of the surviving oeuvre, to give a powerful précis of this astonishing career.

What an artist like Manet rather than Giordano could have understood by “the theology of painting,” the more profound force of this statement, emerges when one contemplates the obvious truth that in the seventeenth century, unlike the nineteenth, the primary subject matter of painting was indeed theology. Painted images had their highest import in the service of religion; they were devotional and didactic in purpose. But there’s a great difference between the painting of theology and the theology of painting. Velázquez offers a glimpse of a strange and haunting idea, one that should have been nearly unthinkable in his own time–that painting could have what we might today call spiritual significance, not because of its exalted subject matter but, somehow, in itself; that painting, through the intensity of its realization, could transmute its subject into something holy.

As odd as this fateful idea would have seemed to Velázquez’s contemporaries (other than Rembrandt), as peculiar as it probably seemed to him even as he entertained it, it’s not one that was ever likely to have had many adherents. Manet, worldly man that he was, would have been leery of putting it so bluntly. Today, most painters would probably find it embarrassing. And yet the relentless urge in modern art to incorporate the most humble and abject aspects of human existence, the most trivial and insignificant objects, surely finds one of its roots here. Duchamp’s ready-mades may amount to a kind of sophisticated joke about this idea, but it’s a dead-serious joke, all the more so for his radical insistence on choice–on being able to see something in a certain way–as a sufficient form of realization. In this sense all the Duchampian postmodernists, no less than their Modernist granddads, are children of Velázquez; that’s why, although Duchamp’s melancholy aperçu that art only stays art for fifty years before declining into art history is generally true, certain paintings by Velázquez strike us as powerfully contemporary.

The first of this exhibition’s four rooms is devoted to Velázquez’s early days in Seville, when he emerged from the informal academy of a prominent local artist who was richer in erudition than in talent, and who was soon to be his father-in-law. But Velázquez must have had more in mind than taking over the family business even as an 18-year-old earning his license, in 1617, as a “master painter of religious images, and in oils and everything related.” Awkward and disjointed as his first independent paintings can be (Three Musicians might be a puzzle put together from unmatched pieces), they possess an eerie power. Caravaggism had swept through Europe, and while Velázquez probably never saw an original work by Caravaggio until years later, his followers’ efforts would have been sufficient to suggest to the ambitious young painter that a way forward might pass through the Italian master’s dramatic naturalism.

Although he was soon painting religious subjects and portraits as well, Velázquez first established himself as a specialist in what were called bodegones, scenes of everyday life set in kitchens or taverns, and in which still-life elements were prominent. (Later on, the term came to refer specifically to still life, but Velázquez never painted a pure still life.) But these are not typical lowlife scenes, such as might be found in Dutch genre painting. There is an uncanny gravity to them, as if there were something more in these people than they might ever know. An old woman cooking eggs looks off into some unspecific distance as if receiving a prophecy. A massive old man selling drinks of water looks like a pope in rags. A maid bends over the crockery on a table looking at it as intently as if it were a supernatural apparition.

Later on, of course, Velázquez would paint popes and prophetesses, and he would see more in them, too, than they could ever know. In fact, it’s extraordinary how often the teenaged Velázquez reached for effects whose true weight he would only be able to show forty years later, when he’d reached the height of his powers. If one misses Las Meninas here, it’s not only in its own right as an étude in transcendental technique, to borrow Liszt’s phrase, but as a fulfillment of what the painter was more crudely aiming at in his first experiments with scenes-within-the-scene, in Kitchen Scene With Christ in the House of Martha and Mary (1618) and Kitchen Scene With Christ at Emmaus, from around the same year. It may be that, as Xavier Bray puts it in the exhibition catalogue, “such pictures helped the faithful to integrate the world of work with that of the spirit,” but in the absence of any obvious allegorical connection between the foreground kitchen scenes and the dreamily sketchy biblical episodes in the distance, one feels rather that the paintings embody a reproach against an absence of integration between daily life and sacred history.

In 1623, thanks in part to his father-in-law’s connections among the Sevillians at the royal court in Madrid, Velázquez was appointed painter to Philip IV–a mediocre king but a remarkable connoisseur. Now the painter of bodegones became a full-time portraitist. Access to the royal collection gave him a new intimacy with sixteenth-century Venetian art, especially that of Titian. The paintings here from the early years in Madrid betray an uneasy experimentation with a variety of styles–a portrait from 1624 of the Count-Duke of Olivares, if it really is by Velázquez, seems to look back to the Mannerism of the late sixteenth century, for instance–but two events helped solidify Velázquez’s mastery. First, the arrival of Rubens in Madrid in 1628, not as a painter but as a diplomat; he snubbed the other Madrid court painters for the duffers they were but spent much time with Velázquez, praising, in Palomino’s words, the “quality and simplicity” of his style. Rubens’s manner had little direct influence on that of his Spanish colleague, but the Flemish painter’s immense abilities, not to mention his standing as an intellectual and man of the world, opened new horizons. So did Velázquez’s first trip to Italy a year later. Not only could he gain new insight into his beloved Venetians, but he could take the measure of established contemporaries like Poussin, Guercino and his countryman Ribera, now established in Naples.

A pair of remarkable history paintings done in Rome around 1630 shows just how far the trip to Italy had taken him. While Velázquez had always been trying to use many different sorts of painterly effects in a single work, the results had always shown a certain disjointedness. In Joseph’s Bloody Coat Brought to Jacob the transition from the play of light and the sculptural solidity of the figures on the left, three of the sons who’ve sold their brother into Egyptian slavery, to the shadowy half-tones that becloud the deceived father on the right is achieved with great suavity. And his employment of rhetorical gesture and facial expression throws down the gauntlet to his Roman colleagues, as if to say, “See how your technique shows feigned emotion more easily than the real thing.” Such posturing had its uses but would play little part in the Spaniard’s mature work, which is emotionally restrained rather than demonstrative. And just to prove the point, what almost amounts to a companion piece, Apollo at the Forge of Vulcan, shows a dramatic moment of exposure rather than deception–the sun god is breaking the news that Vulcan’s wife, Venus, has been dallying with Mars–but entirely without those conventional gestures that lend themselves to insincerity. In both paintings the naturalism Velázquez had practiced a decade before is still at work but now with a sort of classicism subtly subsumed. Part of the paintings’ effectiveness lies in the ordinariness of the models; Vulcan and his helpers look like real workmen, though they lack the grime that Caravaggio might have included, yet as Dawson Carr remarks in the catalogue, their bodies also reflect the artist’s study of classical sculpture. Reality and the ideal, contingent and eternal fuse as they could not do earlier.

Velázquez returned to Madrid in 1631, and except for a second journey to Italy in 1649-51, he remained at court the rest of his life, acquiring new privileges and responsibilities all the while–most notably, becoming the curator of Philip’s ever-expanding art collection. Had he been a less canny courtier we might now possess more of his paintings. Much of the rest of the exhibition consists of portraits, both formal and informal. It’s important to keep in mind that both sorts were part of the job–that the depiction of a court dwarf like Francisco Lezcano, mid-1640s, which probably hung along with several similar works in the Torre de la Parada, a sort of royal hunting lodge, was as much of a duty as the celebration of the King himself, or of his young wife Queen Mariana of Austria as a victim of extreme fashion. Envious competitors among the painters at court made portraiture a reproach; Velázquez, they whispered, only knew how to paint heads. The implication was that his was a servile and literal-minded art able only to reproduce nature, not an exercise of the intellect. Or as we would put it today, insufficiently conceptual. Velázquez is said to have replied: “They favor me greatly, for I do not know that there is anyone who can paint a head.”

Now, the old biographies are full of conventional praise and recycled anecdotes–when Palomino, writing sixty years after the artist’s death, reports that King Philip mistook one of those painted heads for a real courtier, saying, “What, are you still here?” we dismiss it as a cliché. But in the oblique and paradoxical remark “I do not know that there is anyone who can paint a head,” we hear a living voice, one imbued, moreover, with that “phlegmatic temperament” that the monarch himself came to rue in his favorite painter. All the more telling, then, that this voice suddenly emerging from the stately hum of the document is one we are tempted to call modern. Isn’t it strange that in the seventeenth century, we are already hearing of the impossibility of painting, the absurdity of our desire to represent, and precisely from one who had devoted his life to just that? With this remark we find ourselves approaching the world, still unthinkable to Velázquez and his colleagues, in which Cézanne would devote a hundred sittings to a portrait that no one would ever mistake for the person it represents; “If a head interests me, I make it too large,” Cézanne is said to have apologized. Or the world in which Giacometti could muse, “If I could just do a head, one head, just once, then maybe I’d have a chance of doing the rest, a landscape, a still life. But it’s impossible.” And yet between the Spaniard’s cool irony and the Modernists’ anxiety lies a world of difference.

Contemporary artists who are far indeed from anything resembling the Modernism of Cézanne or Giacometti still feel that their undertaking is problematic or self-contradictory in this way. American painter Carroll Dunham recently put it quite simply: “I’m only interested in painting that’s aware of its own dilemma.” For Velázquez, the dilemma was already there, but how many artists have even been able to derive so much concentrated energy from their dilemmas? If anything is missing from this exhibition, it’s a sense of just how restlessly experimental Velázquez remained all his life. It’s astonishing how many “exceptions” there are in this oeuvre–paintings of a sort one would never have expected. Among them is a glory of the National Gallery’s own collection, The Toilet of Venus (1647-51), better known as the Rokeby Venus, part of the exhibition’s final section, “Portraying the Ancients.” For one thing, it’s the artist’s only female nude, a subject Spanish painters of this period were not expected to handle. With an odd sort of hypocrisy, such things were acceptable but only as Italian or Flemish imports; thus was the tension between ecclesiastical condemnation and aristocratic delectation defused. More than that, the goddess’s pose is entirely original, as is the peculiar mixture of exposure and mystery that results from it. The body shown so beguilingly from behind holds an undeniable sensual realism, though understated by the standards of a Titian or a Rubens, but the face glimpsed so darkly in the mirror suggests an even more powerful unknowability. The consummate art, according to Velázquez, is not the one that brings reality close to us as an object of voyeuristic inspection but rather the one that reminds us of our difficulty in grasping it at all.

Although painted a decade earlier than the Venus, Velázquez’s disenchanted Mars (circa 1638) might well represent her implicit onlooker. He is nearly nude himself and, though he faces the viewer, his countenance too is half hidden, shadowed by the helmet that sits on his head so incongruously without the rest of his armor. And yet his eyes pierce the obscurity with their melancholy. Pensive, chin on hand–could this Mars have been a source for Rodin’s Thinker?–he sits on the edge of a disheveled bed, but the tryst has clearly not been a success. Sensuality is seductive but fleeting. Velázquez’s technique was, of course, ideally suited to conveying this sense of transience. Working, in his later years, with an astonishing economy of means, he was able to conjure a sense of the most concrete reality with a few seemingly random swipes of the brush, and yet the illusion dissolves as soon as one draws near. Maybe Luca Giordano was wrong after all. If there’s a theology here, it’s a negative one. In Velázquez’s early years, the holy may have revealed itself in everyday reality, but once he’d learned to see reality as an illusion, where did that leave the holy he’d found in it? He must have been glad not to have had to paint religious subjects any more in the last decades of his life.