One of art history's standard lines is that in the days before photography, the crucial function of a painted portrait was simply to record what someone looked like, for their peers and for posterity.

It's a line I've always more or less bought, though I knew it oversimplified the issue. Now, however, I think that it may simply be wrong. I've recently concluded that, as often as not, it wasn't the face itself that mattered in a portrait. It was the impressive artistic act of painting it. The face didn't have to be real; it just had to be so convincingly painted that it felt real. Commemoration was sometimes just an incidental outcome of an artist's showing off.

This thorough rejiggering of my thoughts came after two days spent with "The Spanish Portrait from El Greco to Picasso," a major show recently launched by the Prado Museum. With this portrait survey, the Prado is hoping to make its first big splash on the international exhibition circuit. Under controversial new management, an institution famous only for its breathtaking permanent collection is being recast as a purveyor of world-class art events, worthy of attention everywhere. Things seem to be on track: "The Spanish Portrait" is, incredibly, the planet's first significant overview of one of the richest, most important and most coherent traditions in Western art.

Since art in Spain was fairly isolated from what was going on elsewhere, the most important inspiration for the country's artists was the art of the Spanish masters who had come before. And since El Greco, Velazquez and Goya, three giants of Spanish and world art, were also major portraitists -- were arguably at their very best in their portraits -- portraiture had a more impressive pedigree in Spain than almost anywhere else. In the 84 important Spanish portraits on show in this exhibition, taken from the Prado's own collection and borrowed from abroad, the museum is offering a rare look at how a single artistic culture dealt with the recording of the human face.

Toward the start of the show, which is mostly hung in chronological order, recognizable faces show up in two so-called donor portraits, finished by Pedro de Campana in 1556. In the later Middle Ages and the Renaissance, the action shown in a religious painting -- in a Crucifixion, a saintly miracle or a heavenly set piece -- would often have a peanut gallery of secular figures looking on from the sidelines. By having themselves included on the edges of the holy scene, a picture's patrons could assert their elevated status, as well as their special access to the magic moment that the picture represents.

In the case of Pedro's two portrait groupings, curators have removed the pictures from their usual place on either side of an altarpiece in Seville's cathedral. Isolated from the holy scene they were meant to flank, the portraits are perfect precursors to all the other family groups portrayed later in the Prado show. Though the function of these donors' portraits may be different from a picture of a prospective royal spouse, or a portrait of a newly crowned monarch, or a record of a saintly missionary, they look pretty much like any of these other convincing portrayals of particular human beings.

This "particularity," the way a portrait seems to pick out one particular example of humanity from among all others, is at the very heart of Spanish portraiture. There is, I think, a fundamental interest in portraiture -- in the simple fact of capturing a single human face in paint -- that transcends the particular use to which a portrait might be put. In the Prado show, functions that seem entirely different produce portraits that look strikingly alike.

The interest in particularity is on show again in a "Hezekiah" painted about 1490 by Pedro Berruguete. Despite its Old Testament subject (Hezekiah was a reforming king of Judah) and old-fashioned churchy treatment (the figure is surrounded by an elaborately tooled gold ground), the picture gets included in this portrait exhibition because of how particular, how portraitlike, it looks. Staring at this middle-aged Hezekiah, with his strange ears, weak eyes, wrinkled brow, hairy mole and incipient double chin, you're sure that he's been modeled on some particular living man, and not on a generic idea of how aging men tend to look.

But if there's one thing this exhibition teaches, it's that a portrait can seem to commemorate a particular person even when that can't have been its goal. Viewers didn't really think Berruguete had caught the features of a long-dead Jewish king. They just demanded pictures that could make it seem as though he had. They demanded pictures that worked in a snapshot mode, regardless of whether there was some real subject to be captured for posterity.

Spanish painters were almost always eager to give their figures striking individuality, and they were very, very good at it. So there's no knowing, just from looking, if a picture is truly meant to preserve its sitter's features -- which is the standard "proto-photographic" assumption made by art historians when they come across a "lifelike" picture. The painter may simply be using those features -- perhaps borrowed from the studio handyman or a hired model -- to make his fictional figure more lifelike. The remote King Hezekiah comes to seem more fully present to the Renaissance artist's audience if he looks as though he is just another one of them -- and that's true whether he recalls a known person's face or not.

A figure's features may not in fact be modeled on any single sitter at all. A skilled painter could assemble an identikit of notably "lifelike" features -- of hairy moles and jowls and 5 o'clock shadows; of bee-stung lips and cleft chins and peculiarly long lashes -- and then recombine the parts at will to give a face the look of individuality, without needing an individual to pose for it.

It's that very special, constructed, almost rhetorical notion of lifelikeness that is the thread that runs through this show, and through almost all of Spanish portraiture, whatever its function. It's not accuracy that matters in these pictures. And it's not even expressiveness: Many of the sitters' characters are impossible to read with any certainty. It's just an idea that someone, somewhere, could have had -- must have had -- the peculiar combination of features and foibles and idiosyncrasies that each portrait shows. A picture needs to be convincingly life-like. Whether there is some particular real-life that it is like is almost beside the point.

Lifelike idiosyncrasy is a crucial feature of almost all of El Greco's portraits. If weirdness got the painter into critical hot water in his narratives and sacred works, it won him praise in his portraits, which were the most appreciated and by far the most influential aspect of his art. (When Velazquez died in 1660, his estate included several El Greco portraits, by then many decades old.)

All of El Greco's sitters look like characters, rather than just stuffed heads, because of the peculiarities he piles onto them: Eyes are almost never quite the same size or the same height; fingers can be elongated, even clawlike; ears can be so huge they almost flap, or pinned back so that they all but disappear; gazes are often strangely askew, or askance, or remote -- anything, so long as they aren't neutral.

I'm not saying that El Greco's portraits aren't accurate renditions of their sitters; they were widely praised for being so. I'm just claiming that there is a special, extra quotient of everyday peculiarity that El Greco throws in, to make his portraits count as notably lifelike.

It's this artistic impulse toward lifelikeness -- the pretense that some detail has been culled from the passing scene -- rather than the actual recording and commemoration of true facts, that I think lies behind all Spanish portraiture. This kind of interest in the everyday is usually known as "genre" painting: It's this interest that accounts for the popularity of "genre" pictures of lowly tavern scenes and butcher's stalls and prostitutes.

In Spanish art, almost every portrait, from the king on down, becomes a genre painting.

The Spanish portraitist's leanings toward genre, hinted at in Berruguete's "Hezekiah," is full-blown in similar pictures by Velazquez. His images of ancient philosophers and sages could as easily be paintings of local riffraff. His disheveled "Aesop" looks more like a rag-and-bone collector than like one of antiquity's most esteemed writers and wits; if it weren't for the picture's prominent caption, you'd be more likely to imagine him selling the book under his arm for scrap than filling its pages with words. The face is perfectly portraitlike and explains the picture's inclusion in the show: It's full of the particularity we think of as singling out one person from another. But the picture is clearly not about commemorating some real, humble 17th-century individual; in a sense it does precisely the opposite, hiding the true sitter's identity behind the persona the artist has imposed on him.

The famous Velazquez paintings of the dwarves and jesters at the Spanish court work along the same lines. They aren't meant as loving records intended to preserve their sitters for posterity; it's almost accident that a few of the subject's names have come down to us. They are really meant as genre pictures, which record striking details of the everyday world and, by doing so, show off painting's astounding ability to make such records.

But those dwarfs aren't your standard genre pictures, either, because they go one further than such pictures usually do. They also manage to have the trappings at least of respectful commemorative portraiture. Velazquez makes the confusion of categories go both ways: If El Greco injects the striking peculiarities of genre painting into his worshipful commemoration of friends and patrons, Velazquez takes full-blown genre scenes, full of tawdry freakishness, and gives their subjects dignity and the semblance of status.

I'm not sure the gesture comes from any special warmth of character and humanity in Velazquez, though that's the standard claim. I think the gesture is more likely to have appealed to the refined artistic sensibilities of patrons. They would have valued these paintings as examples of Velazquez's brilliant wit in blending portraiture proper and genre -- in taking the humblest and most peculiar of their lowly paid retainers and doing them up as credible simulations of real high-class human beings.

Patrons must have equally appreciated the big dose of genre Velazquez injected into portraiture, even when he was depicting royalty. His most famous painting, "Las Meninas," is really a genre picture that happens to have noble figures in it. It pretends to show a snapshot glimpse of life at court: The royal princess, attended by ladies-in-waiting, dwarfs and dogs, has stopped by to watch Velazquez, the royal portraitist, as he paints a picture of the king and queen. (The picture's subject is actually much more complicated and ambiguous than that, but it will do as a bare-bones reading.) All the painting's many genre details, however, are really just window dressing for what turns out to be a portrait of her little royal highness.

The same process applies even in some of the more straightforward royal portraits of Velazquez.

In one painting begun in 1635, a 6-year-old Hapsburg prince is shown out hunting with his dogs, as casual as any other countrified boy. In a picture of the boy's father, Philip IV of Spain, the king can be reduced to just an unimpressive face, with one droopy eye, a lantern jaw and a jutting lower lip. What's really on display in a portrait like this, I would argue, isn't regal presence and nobility, or even the royal mug. It's raw artistic skill -- the sheer ability of a great artist to render particularity, regardless of the sitter. Even a royal face, stripped of finery and isolated on a black background, achieves a sort of genre effect: The painter hasn't built a fancy picture; he's just captured one instant from the passing flow.

And then, as a kind of culmination to the entire Spanish portrait tradition, we get Francisco de Goya. (The two rooms that follow Goya's in this show, which take the story right up to Picasso, feel like postscript. They're needed to round the story out, but we already know the whole plot before we get to them.) If Velazquez was willing to put elements of genre into his classy portraiture, Goya was willing to throw class completely out: His fancy sitters often look as much like bakers, fishwives and night watchmen as like the top end of Spanish society. What Velazquez did to Aesop, Goya does to paying patrons, sometimes taking genre painting to its very furthest extreme, where it becomes caricature.

You look at Velazquez's little princely huntsman and feel that he looks surprisingly poised and self-possessed for a child at play. You look at Goya's picture of the adult King Charles III, also shown outside with gun and dog, and you think a goofy gamekeeper has been dressed up in clothes above his station.

"Las Meninas" shows how, even in a scene snatched from everyday life at court in 1656, the Spanish royal family achieves surprising dignity and grace. Goya's "Family of Charles IV," from 1800, imagines that the unvarnished everyday, even for the richest and most powerful, is almost always grace-free. Queens can go a bit to seed, like any one of us; a king can be a stuffed shirt hung with medals; a gathering of siblings and relations, even when they're royals, will always turn out to be a pile of strange characters, their familial dysfunctions plainly on display.

That's life, folks, and a painter's job is just to capture it, says Goya. And he gets away with it because he's Spanish and has a long line of other Spanish masters who softened his patrons up for him.

Centuries of Spanish painting had established the idea that the greatness of a portrait, whatever its function -- whether it shows a biblical king, a half-mythic ancient sage, a living dwarf or reigning royalty -- lies in how well it renders the look and feel of the plain truth. And once that has been established, Goya's patrons have a simple choice: To get a tough picture that's a demonstration of an outstanding painter's skills, or buy a hackish piece of flattery. Amazingly, a good number of them, like their many predecessors over the years in Spain, chose the masterpiece.

But then, it's always been said that great art does most credit to the person buying it. So you could argue that the worse that Goya's portraits make his Spanish sitters look, the better those patrons come off in the end.

One country's notions of impressive portraiture: Clockwise from above, Goya's "Charles III as a Hunter," circa 1788; El Greco's "Gentleman With Hand on Chest," from 1578-1580; and Velazquez's "Aesop," circa 1638.Goya's "Family of Charles IV," from 1800, imagines that the unvarnished everyday, even for the richest and most powerful, is almost always grace-free.Pedro Berruguete's "Hezekiah": Modeled on some particular living man, not the biblical character. Left, Velazquez's "Las Meninas":

A genre picture peopled with noble figures. Velazquez's "Prince Baltasar Carlos as a Hunter," from 1635-1636.