ART / First up against the wall: Andrew Graham-Dixon on Tapies at the Serpentine Gallery and Manet's The Execution of Maximilian at the National Gallery

Leonardo da Vinci once said that whenever he was stuck for an idea he would go out and stare at 'a wall covered with dirt'. In the chance configurations of stained, crumbling stucco he would find 'landscapes, battles, clouds, uncommon attitudes, humorous faces, draperies. Out of this confused mass of objects, the mind will be furnished with an abundance of designs and subjects perfectly new.'

Leonardo was not lacking in foresight, but even he probably never suspected that one day artists staring at the dirt-covered wall would see, not a random source-book of ideas, but an aesthetic ideal - a prompt for paintings that would, themselves, be as tough and sullenly ambiguous, as lacking in perspective or apparent coherence, as chunks of weathered masonry. For them, the found patterns on urban walls would exist not to be reconfigured, not to be reimagined into the grander forms of heroic figurative art, but to be replicated in all their dumb, prosaic solidity.

The Spanish artist Antoni Tapies holds the superstitious belief that 'our names influence our characters and our particular destinies'. It is not hard to see why. His own name is Catalan for 'walls', and Tapies has been making paintings that look very much like walls for almost 40 years. The Tapies exhibition currently at the Serpentine Gallery is devoted to his first essays in wallness, mostly from the 1950s. These are the paintings which, when first seen in New York in 1959, were sniffily described by a reviewer for ArtNews as 'large cumbersome statements in dun colours, like sections of palaeolithic subway removed from environments where they might excite a passing interest'. That is a pretty good account of them, although allowances have to be made for American critical jingoism. The young Tapies's art is still impressive in its reluctance to please, in the sheer brutality of its forms.

The paintings are heavily worked and heavily worked over. Tapies's modus operandi was to make a picture and then assault it. Large Matter with Grey Papers is a heavy, characteristically dark abstract whose burned-out textures and sooty indecipherability suggest that it was, at one point in the creative process, set on fire. Grey Painting is a dense slab of a work whose roughened surface has been comprehensively vandalised: slashed and scratched, scarred and abraded, it is the sort of thing Freddy Krueger might be expected to produce were he to take up art as a recreational activity. Occasionally, in Tapies, something like an image struggles into visibility: a ladder leading nowhere in particular; a spectral figure; an empty chair. But the artist seems as concerned to obliterate, to cancel or negate, as he is to indicate.

Tapies's wall-like paintings have their own history. They look back, for one thing, to Surrealist art. Before Tapies, the Surrealists had seen, in the mouldering city wall, a model for the workings of the subversive imagination. Its indeterminate blobs and blotches, its chance collisions of ambiguous, suggestive, fantastical forms, seemed like perfect examples of automatically produced imagery - and the Surrealists were particularly keen on 'automatism', on the liberation of the image into new psychic realms supposedly made possible by a suspension of the conscious will.

The randomly defaced wall, with its coarse textures interrupted by graffiti, by bits of peeling flyposter, stains and scratches, was a natural inspiration for certain forms of Surrealist painting: Miro's flat walls of colour, or the rough monochrome surfaces of Klee's drawings, places where graffiti-like biomorphs come out to play.

Klee and Miro were sources for Tapies, but he was even more powerfully influenced by a different modernist mythology of the wall, whose chief incarnation was to be found in the urban photographs of Brassai. Brassai's black-and-white pictures of graffitied walls in Paris memorialised what the photographer thought of as the secret mental life of a community. The walls of the city were an arena for covert political dissent, for the furtive expression of public discontent with the status quo - especially so in the Paris under German occupation which he photographed. Brassai wrote that 'the wall is a refuge for everything that is repressed, rebuked, forbidden, oppressed'. It was this aspect of the wall, more than any other, which came to preoccupy Tapies.

Tapies' early works are unusual because they are abstract paintings that harbour political intent. They contradict the general belief - shared by Hitler and Stalin, among others, who had their reasons to be interested in the subject - that abstract art is incapable of coherent political statement. Tapies, echoing Brassai, once said of his paintings that they 'were related to street graffiti and to an entire world of repressed, clandestine protest that was circulating on the walls of my country'.

Tapies's early works are themselves clandestine protests against the Francoist regime that governed Spain in the aftermath of the Spanish Civil War. They represent an ingenious solution to the problem of how to make anti-state art while under the surveillance of the state. Tapies's thick, scarred walls are history paintings of a kind, even if the histories they relate are imprecise, evoked rather than spelled out. The innocently titled Large Painting with Dotted Lines resembles a wall riddled with bullet holes, mute witness to a society ruled by the firing squad. Crucified Form, with its vestigial suggestion of a brutalised male torso, suggests another kind of wall, interior rather than exterior: the wall of an interrogation chamber, perhaps, a place of secret torture and death.

In both cases, of course, there is nothing that could be said to convict Tapies of anything as concrete as a political opinion, definitively expressed. He might have defended himself, to the Francoist censors, by saying that Large Painting with Dotted Lines was merely a formal exploration of certain abstract shapes, and that Crucified Form was a modern variant on an ancient and honourable theme. But still their sense is plain enough.

Tapies's paintings are demonstrations, among other things, of an old truth about art, still insufficiently acknowledged: context is meaning. Cumulatively, Tapies's early works (he has done little to compare with them since, becoming, like many artists of his generation, a mannerist of his own devices) amount to a metaphor for a national predicament. No wonder, maybe, that Americans in the late 1950s found them disconcerting, ugly, cheerless works. It is hard to imagine anything much further removed from the hedonist expanses of Colour Field painting, or the mute icons of Pop, than these solemn, reproving abstracts.

Tapies might be said to have given new life to a tradition of dissenting political art that has its roots in Goya's The Fourth of May and culminates in Picasso's Guernica. It is a largely Spanish tradition, and another artist to have played an active role in this mini-history of modern, politically engaged painting, Edouard Manet, was a Frenchman unusually influenced by Spanish art. The three paintings most directly responsible for this aspect of his reputation, Manet's varying treatments of The Execution of Maximilian, are currently on view at the National Gallery. It is the first time that these pictures, which were censored from public view by the French authorities during Manet's lifetime, have been seen together.

The story that Manet chose to record was fairly straightforward: the death of Maximilian, a minor Habsburg suddenly installed on the throne of Mexico by Napoleon III to further French imperial ambitions and as suddenly abandoned, at the hands of Mexican Republican forces. Manet found two ways to paint the shamefulness of it all. In his first version of the subject, the emperor and his two generals, almost completely obscured by gunsmoke, are shot at point-blank range by a firing squad of anonymous sombreroed Mexicans: an image of pointless, brutal death, brutally rendered.

Then, perhaps finding such a solution too extreme, too thoroughly ambiguous, Manet hit on another, more pointedly critical, formula. His second and third paintings (the second, partly destroyed, is familiar to visitors to the National Gallery, which owns it) envisage an altogether different kind of death, a death whose keynote is not jarring obscurity but indifference. Manet's real subject is not the emperor himself - his figure has been all but lost in the National Gallery's damaged second version and he is a curiously ghostly presence in the third - but the firing squad whose job it is to kill him.

What a fine job they make of it; how coolly they go about their work (and how coolly and beautifully, in their smart grey and black uniforms, they have been painted). They could be on the practice range, for all the emotion they display. Their epitome, the NCO who stands a little to one side, nonchalantly cocks his rifle to fire another shot should it be needed.

You could say (and it has been said) that Manet, here, was involuntarily incriminating himself - that he was showing himself up as the painter whom Theophile Thore criticised for 'placing no higher value on a head than a slipper', and of whom Paul Mantz wrote that 'life's spectacles do not move him'. But there may be more to it than that. Manet's subject, after all, was a death that had been caused by French indifference, by Louis Napoleon's fairweather-friend reluctance to send aid to his own puppet emperor when the chips were down. How better to convey this than by painting a picture of Maximilian's death in which the executioners (whose uniform, incidentally, closely resembles that worn by the French Imperial Guard) are seen as the embodiment of indifference, of ruthless sang-froid?

Political art may always be most effective when least strident. Certainly, the very different political art of Tapies and Manet, united only by a consciousness of the force of subtle, circuitous strategies of protest, would seem to confirm this. Convict your enemies, but do so indirectly, ironically. It is more damning that way. And you're less likely to get thrown into jail.

For details of both exhibitions, see Listings, opposite.

(Photograph omitted)