This is a curiously downbeat title for the electrifying exhibition that opens tomorrow at the National Gallery. Borrowing from the literary critic Harold Bloom, I'd have called the show "Dutch Portraiture and the Invention of the Human".With only 60 paintings (all but a few of the highest possible quality), the organisers take us from a late medieval world in which humanity is defined by outward appearance and social status to a modern age in which men, women and children are seen as unique individuals, each a complex personality with foibles, contradictions, and unexpected depths of character.The explanation as to why this happened in art at the time and in the place that it did is fairly straightforward. In 1579, the Dutch Republic broke free from Spanish Hapsburg rule, transferring power and patronage from court and church into the hands of a newly liberated Protestant middle class, who had plenty of money and only themselves to please.You can see the change take place before your very eyes at the beginning of the show. The first pictures we see are two portraits of Albert Sonck and his wife, each with one of their two children at their side, painted in 1602 by Jan Claesz. Although the faces of the young couple are astonishingly lifelike, each is shown in a fully frontal pose, as stiff and as hieratic as a figure on a playing card, their bodies flattened like giant insects pressed under a plate of glass.Only 20 years later, in Frans Hals's double portrait of Isaac Massa and his wife Beatrix van der Laen, everything has changed. It is not just that the sitters are now surrounded by space, air and light, but that despite his cloak, her voluminous dress, and the hat, cap, collars, cuffs, ruffs, stockings, shoes and gloves that cover their bodies from head to toe, this is a couple who have a blatantly happy sexual relationship.At least that's the way I read Beatrix's knowing look and sexy, satisfied smile, and why, as Isaac leans back with one hand over his heart, she drapes her arm over his shoulder in an easy, intimate way. Though husband and wife bend in opposite directions, they seem to be joined at the hips and thighs, one trunk with two branches. Unlike the husband and wife who are separated in Claesz's pendant portraits, these two people not only share the same canvas, they sleep in the same bed.Although more conservative Dutch painters such as Nicolaes Pickenoy continued to paint exquisitely refined portraits during the first half of the 17th century, it was primarily those of Hals and Rembrandt that struck this new note of informality. Think of the portraits Velázquez painted of the Spanish court, or of Van Dyck's of the Genoese aristocracy, and you see how innovative their approach to portraiture could be.And so, Hals twice paints the merchant Willem van Heythuysen, and each time we see a different - and not always sympathetic - side to his personality. In a magnificent full-length portrait that we know hung in the entrance hall of his Haarlem house, Heythuysen strikes a gallant pose, one hand on his hip like a Spanish grandee, a swag of red drapery behind him and a huge sword in his right hand.Since he was neither a soldier nor an aristocrat, there is something almost comical in the portrait's pretentiousness. Here is a Malvolio, pretending to be what he is not. But in a second, small-scale portrait the same man is shown in riding kit, flexing his riding crop and tilting his chair backward in a way that is unknown in Dutch formal portraiture at this date but fairly common in genre scenes, especially ones showing boors in taverns. Clearly the client drew a distinction between his public and private self - how he wished to present himself to the world, and who he was when alone in his study.All human life is here. Salomon de Bray paints newborn twins in their golden cradle, surrounded by a foaming sea of linens, blankets and draperies. When Frans Hals painted little rich girl Catharina Hooft, he contrasted her stiff dress embroidered in gold with the simple attire of her loving nurse. But he also showed that the personality of the lively two-year-old was as forceful - and as good-natured - as that of the woman who looked after her.I sometimes find Rembrandt's late portraits too self-consciously empathetic for my taste, but in this show the Falstaffian look of bewildered disillusion in the eyes of a corpulent old man, and the stoically borne grief in the eyes of the widow Margaretha de Geer pierce the heart like a needle.Dutch painters extended the very definition of what portraiture could be. In The Anatomy Lesson of Dr Nicolaes Tulp, Rembrandt turns a static group portrait into a narrative painting by showing the doctors responding with varying degrees of interest and understanding to what is being said and shown - some craning to see the flayed arm of the cadaver, others looking at the lecturer, and still others turning to look at us.(In The Anatomy Lesson of Dr Nicolaes Tulp, Rembrandt turns a static group portrait into a narrative painting)In Rembrandt's late masterpiece, The Syndics of the Clothmakers' Guild, it is as though we have just interrupted a meeting in progress. One syndic rises from his chair at our approach; others look up from their work with polite interest or mild indifference. And Rembrandt wasn't the only artist to conflate portraiture with other genres. In one of the most appealing works in the show, Jan Steen's portrait showing the orphan Bernardina van Raesfelt feeding her pet lamb, the artist combines portraiture with landscape, animal, bird, and genre painting.The majority of the people portrayed by these artists wear black, but don't let that put you off: what a black it is! Not dreary Calvinist weeds but the rarest imported silks, richly dyed and embroidered, trimmed with the most expensive lace and worn with gold, pearls and diamonds.And, when the Dutch decide to "do" colour, they pull out all the stops. Just look at Johannes Verspronck's portrait of the Haarlem officer Andries Stilte dressed as a standard bearer in a costume of mauve-pink satin trimmed with a blue silk sash, his lace collar tied with a bow of soft green, his wide-brimmed hat flamboyant with feathers of blue, yellow and red. Stilte carries it off - just about.But then as now, there were also fashion victims. We should all spare a thought for the poor young man painted by Gerard ter Borch in the latest French fashions of conical hat, short cloak, and petticoat breeches with giant ruffles. Here is a sad sack who desperately needed advice from the 17th-century equivalent of our own Hilary Alexander.So far, I've stressed the broad social and cultural contexts against which these pictures can be viewed. Now forget about all that. Don't think, just look. Simply in terms of technical virtuosity, these artists take your breath away. Focus on details such as the full-length figure of the militiaman at the far left of Hals's vast group portrait The Meagre Company miraculously built up in tones of silver-grey shot through with light blue and white, then virtually wrapped in a flag and sash of brilliant orange.In Rembrandt's portraits of the syndics of the Clothmakers' Guild, it is the interplay between layers of paint in the patterned table covering that is so fascinating, that and the way Rembrandt renders the texture of the flesh on each face differently - soft for one, rugged for another, pockmarked for a third.In Rembrandt's Portrait of an Elderly Man, the loose strings of the open collar are drawn by dragging a dry brush loaded with white paint down the black coat in a single stroke, without losing contact with the canvas. And, in Hals's newly restored portrait, we can now see that Aletta Haneman's skirt was created by broad strokes of mauve and pink paint, like shot silk.