In 1938, bullied by Ben Nicholson, John Piper was struggling to be an abstract painter. Plagued by doubts, he joined the crowd of 15,000 queuing to see Picasso's Guernica in London. The picture, said Piper's wife, "acted upon him like rape". After that, there was no more abstraction for Piper. At 34, his world had been shaken by a Spaniard nearing 60, a man who had been shaking worlds since before Piper was born. Even now, even here, Picasso could pull cats out of bags.
Twelve years before, another young painter had travelled to sit at Picasso's feet. Salvador Dalí was Piper's age, although there similarities end. If Piper would feel raped by Picasso, Dalí's seduction was entirely voluntary.
Arriving in Paris in 1926, he went straight to Picasso's studio and declared: "Master, I have come to see you before seeing the Louvre." In Dalí's telling, Picasso nodded sagely and said: "You're quite right." A decade before, the young Joan Miró had also turned up on Picasso's Paris doorstep. He had tried to meet Picasso, back home in Barcelona, by calling on his mother when he knew her famous son to be staying: Picasso was out. Now, in 1917, Miró arrived with a home-made cake, an offering to the god of art.
Like God, Picasso would have had to be invented if he hadn't existed. To Dalí, the irrationality of the older man's Cubism would make a nifty starting point for that exploration of the Freudian unconscious which underpins Surrealism. For Miró, Picasso meant something more generic – the courage to look forward rather than back, maybe. For both men, Picasso marked an escape from the cloying classicism of Spanish (and particularly Catalan) art. Most of all, he was a myth, a fiction with a Paris address.
Art history and gallery curators tend to like their facts straight. It is a brave show that celebrates untidiness, and Picasso, Miró, Dalí at the Palazzo Strozzi is one. Starting with Dalí's meeting with Picasso in 1926, it works backwards, room by themed room, to 1907 and the extraordinary Cahier 7 – the rarely seen notebook in which Picasso sketched the figures for Les Demoiselles d'Avignon. Miró was a decade older than Dalí, Picasso a decade older than Miró. Although this exhibition is subtitled The Birth of Modernity, there is no baby, as such. Instead, the Strozzi's curators liken the show's structure to a film whose outcome is explained by a series of flashbacks and prequels – Dalí's Neocubist Academy, Miró's Still Life (The Carbide Lamp). Woven in with all this is an Oedipal tale in which Picasso rejects his own father only to be adopted by these two murderous young sons in art. At the end – which, in chronological terms, is the beginning – you are left thinking ... well, what?
I'm willing to believe that coming out of a show scratching your head is a good thing. Dalí was a well-known mythomane, and it is likely that his 1926 meeting with Picasso never happened. If so, the plot of Picasso, Miró, Dalí begins with a lie. In a film – Michael Haneke's White Ribbon, say – doubts as to whether the key event ever actually took place lends a useful uncertainty. I'm not sure this translates well into the curating of exhibitions, though. I paid two long visits to Picasso, Miró, Dalí, and enjoyed both, although I'm still wondering what I was meant to take away from the show, other than a sense that art history is odder than you'd think. Or, of course, not.
This is certainly not the problem with the Van Gogh Museum's Picasso in Paris 1900-1907. The myth of Pablo holds that he arrived in Paris fully formed, sweeping all before him in a wave of raw Spanish genius. Common sense and this exhibition tell us that that can not have been the case. Picasso in Paris patiently examines the kind of things a talented 19-year-old from a relatively parochial background might have found in Paris, and how he might have dealt with them. Thus Picasso's Café Concert Singer looks a lot like Toulouse-Lautrec, while The Dwarf Dancer has echoes of Van Gogh and Woman in Cap of Gauguin. And so on.
The shortcomings of this "and then" approach are underlined by the show's unpicking of Picasso's debt to Symbolism. There isn't one, really: what matters is not what he took from Puvis de Chavannes but what he so quickly rejected. Put another way, the compelling things about Picasso are the ones that cannot be explained. If Picasso, Miró, Dalí is over-ambitious, then Picasso in Paris 1900-1907 tends the other way. Both suggest that the impulse to tie Picasso down is as strong as ever. Both confirm that it remains impossible to do.
'Picasso, Miró, Dalí', Palazzo Strozzi, Florence (00 39 055 2469600) to 17 Jul. 'Picasso in Paris 1900-1907': Van Gogh Museum, Amsterdam (00 31 20 570 5292) to 29 May
Charles Darwent spins a yarn with Alfred Wallis at Compton Verney
Anish Kapoor gets a rare out-of-London showing as various major collections, and the artist himself, lend work to the Manchester Art Gallery (till 5 Jun, then touring to Nottingham and the Yorkshire Sculpture Park). Catch the British Museum's, Afghanistan: Crossroads of the Ancient World, with its treasures feared lost to looters (till 3 Jul).