All becomes clear when you know that this is the portrait as retrospective self-justification: it was commissioned by Lady Digby's husband to put down rumours that she had had an affair with the third Earl of Dorset. What Van Dyck painted was the Victory (laurel wreath) of Lady Digby's Chastity (doves) and Wisdom (serpent) over the forces of Lust (Cupid) and Deceit (Janus).
Not that any of this makes the picture seem that much more approachable to modern eyes. It is a little hard, really, to imagine a modern equivalent. David Mellor as Fidelity Triumphant? , maybe ('the sitter, portrayed in a rocky landscape, is accompanied by his dog, symbol of constancy. The significance of the discarded clothing with which the landscape is strewn, which has been identified as the strip of Chelsea Football Club, has no iconographical precedent and has baffled most commentators . . .') It would never happen. These days, not many people who want to justify or reinvent themselves hire painters. They hire ghost-writers and try to sell their Exclusive Stories to The Sun.
One of the traditional fascinations of portrait painting is the illusion it gives of contact with the past. Looking at a Holbein, say, it is possible to think of the picture as a sort of time machine, capable of transporting you back to the court of Henry VIII. But the swagger portrait is consecrated to the projection of fictions. The paintings in the Tate's show do not, strictly speaking, depict those who sat for them. What they depict is, rather, those people's fantasies of themselves.
Van Dyck's elaborate portraits of Charles I and his court have retrospectively acquired a benighted, melancholy air. Time has ironised their symbolism, has made these images - of Olivia, Wife of Endymion Porter as an Arcadian wood nymph, or A Lady as Erminia, Attended by Cupid - look like frail, ludicrous impositions of fantasy on reality. What makes Van Dyck's pictures poignant is the sense that these people really believed in their own allegorical fictions of the self: they believed they were gods and goddesses of a kind, inhabiting a neo- Platonist heaven on earth into which they had been raised, by divine right, above the common people. They were wrong: Charles I would be executed, and the whole elaborate work of art that was his court would collapse.
You could argue, on the evidence of this show, that the swagger portrait begins to die almost as soon as it has been born. Numerous portraitists after Van Dyck would cater to the theatrical self- images of their sitters, but there is, nearly always, a simultaneous disclaiming of complete seriousness, and a sense that the masquerade is not entirely in earnest. The swagger portrait becomes a form which characteristically strikes a note of witty, self-conscious fraudulence.
When Lely paints Barbara Villiers, Duchess of Cleveland in the mid-1660s he creates the first unequivocal sex symbol in British art: she gazes out at the viewer with languorous seductive intent. Yet Lely has also given his sitter the symbolic attributes (sword and palm branch) of Saint Catherine. These fancy dress details seem meant to provoke a wry smile: painting the most notorious of Charles II's mistresses (bar Nell Gwynne) as a Christian saint, Lely suggests his own, sardonic attitude to the allegorical impostures of Van Dyck.
By the late 18th century, when Reynolds paints the three Montgomery sisters as Three Ladies Adorning a Term of Hymen, grand manner portraiture in the Van Dyck mould has become a weird, jokey game of charades. Reynolds' three aristocratic subjects have assumed roles in a bizarre tableau vivant. Reynolds has painted them, in effect, in the act of impersonating another painting: they are playing at being in a Poussin bacchanalia. The picture is a risque fantasy, turning English aristocrats into pagan nymphs. But it is a fantasy crucially disarmed by its own witty self-consciousness, which is a form of disclaimer. Only joking.
By the beginning of the 20th century, when the swagger portrait finds its last great exponent in John Singer Sargent, its original sense of grand theatre has been so dissipated that it only survives in the paint itself - Sargent's miraculous but heartless genius for pastiche. This is what he uses to dress his sitters up, aggrandising them by making them seem as if painted by Velazquez, or Hals (or Van Dyck). Sargent's handling itself is the theatrical mise-en-scene. And maybe this is where the swagger portrait really breathes its last, smothered by the weight of its own self-consciousness.
And maybe not. There is another version of the history of the swagger portrait. There has to be, because there is another kind of swagger portrait which hasn't been mentioned yet. It, too, goes back a long way (although not quite as far back as the time of Charles I), but it conceives the project of idealising the self in completely different terms. Its exponents are not van Dyck and Lely and Reynolds and Sargent, but Hogarth and Raeburn and the Victorians. The presence of this alternative tradition of swagger gives the Tate show the character of an argument as well as a history - a long debate, conducted in painting, about what it should mean to be British.
The icon of this alternative tradition of swagger is Hogarth's portrait of Captain Coram, that great testament to the virtues of the self-made man. What Hogarth's painting suggests, when you see it close to the works of Van Dyck and Reynolds, is a thoroughgoing disdain for their theatres of the self. Hogarth's painting, which places the sitter in his study, surrounded by books, against a backdrop of the sea glimpsed through a window, is not without a certain rhetorical grandeur - but its rhetoric declares that this is not a man dressed up to seem like someone else, but a man who is simply, solidly, there. This streak of solemn, anti-theatrical, empirical grand manner portraiture is continued by Raeburn and also, notably, by the Victorians, who share Hogarth's fundamentally bourgeois conception of moral heroism.
'The Swagger Portrait' opens out, from the narrow compass of a single genre of painting, into one of the largest areas of debate within British art and culture of the last three centuries. The conflict between Van Dyck's and Hogarth's versions of the portrait does not merely expose two conflicting fantasies of the self - it also takes you to the heart of an old argument about the ways in which foreign influence can corrupt or endanger the British national character.
Van Dyck's aggrandisements of the self were seen, by Hogarth and his followers, as a decadent imposition of foreign, continental Catholic values on the true, dependable, Protestant British soul: adapting the Catholic language of religious painting to portraiture, Van Dyck was seen to have brought an alien sense of theatre to the British and instilled in them a corrupting fondness for fancy dress. Hogarth, on the other hand, embodies that strain of British Protestantism which has a profound, adversarial distrust of the theatre in all its forms, and which holds that to act, to play a part, to reinvent the self by putting on a costume, is not merely hubristic but deeply immoral. Despite Shakespeare, there is a strong tradition of hatred of the theatre within English literature - a tradition of ascetic Protestantism that runs from Milton's Paradise Lost (where Satan is described as an actor) to Jane Austen's Mansfield Park (where a willingness to take part in amateur theatricals is a sign of moral weakness) and beyond.
All of which suggests that the tragedy of modern British portraiture may not be, exactly, that the swagger portrait died out - but, rather, that the more worthy, less adventurous, Protestant version finally won the day. The modern boardroom portrait, in fact, may represent the cul-de-sac, the last resting place, of the Hogarthian tradition of anti-theatrical portraiture.
It is also possible that the other, Van Dyckian tradition of theatrical portraiture, the portraiture of self-reinvention, did not die out but turned into something else. There is still a portraiture whose subjects are members of an elite rather than of the bourgeoisie; whose subjects are free to metamorphose in theatrically extravagant ways and who require glamour, sexiness, desirability to be conferred on them; a portraiture that tempers all such fantasies by having its tongue in its cheek, by declaring its own knowing, self-conscious fraudulence. It is the portraiture of rock stars. The Van Dyckian swagger portrait may not be entirely alien to us: it lives on, you might say, in Madonna's Sex.