The Art Festival & The International Festival, Edinburgh

Take a stroll around this city-wide festival and discover Darwin's words in white neon, Agnes Martin's quiet power and the Wilson twins at their best

For reasons not entirely clear to me, the visual arts component of this year's Edinburgh Festival has been split between the Edinburgh Art Festival (EAF) and the Edinburgh International Festival (EIF).

Thus Unfolding the Aryan Papers, a film by Jane and Louise Wilson at the Talbot Rice Gallery, comes under the remit of the EAF while Joseph Kosuth's An Interpretation of This Title, 20 feet away in the same gallery, is EIF. Likewise, the National Galleries' Enlightenments show is EIF while its Agnes Martin exhibition, in the Artist Rooms series, is EAF. Oh, and although it's right across town from the Dean Gallery, the Kosuth show is part of Enlightenments, which is ... um, well, you get the picture. Since host establishments are also publicising shows, the opportunities for confusion, always numerous at the Festival, are multiplied this year. So draw up a battle plan before you go.

But do go. The Wilsons have faded from British view lately, and their film marks a welcome return here. Based on interviews with an actress hired by Stanley Kubrick to play in an unmade movie about a Holocaust survivor, Unfolding the Aryan Papers (to 26 Sep) is the Wilsons at their finest – a probing of that gap between probability and possibility which the sisters, identical twins, have made their own.

Kosuth's installation also plays with gaps, this time between the evolutionary theories of Charles Darwin and the survival-of-the-fittest perversion of those theories by Friedrich Nietzsche. In the lower part of the Talbot Rice's Georgian Gallery – a room where the father of evolution once studied – Kosuth has copied Darwin's notebook doodles in white neon: in a niche at the room's far end is the famous tree-of-life sketch with the words "I think" ... in Darwin's hand. In the room's upper half are hidden away the logical conclusion of Darwin's thoughts, in the form of Nietzsche's will-to-power apophthegms, among them the historically ominous words: "Creating – as selecting and finishing the selected."

As I say, An Interpretation of This Title is part of an Edinburgh-wide EIF exhibition called The Enlightenments (to 27 Sep), which has its epicentre at the Dean Gallery. (En route to this, you can take in the show's third part – Susan Norrie's apocalyptic mini-movie, Enola – at Collective in Cockburn Street (to 26 Sep). Typically, Collective is also showing a non-EIF work by Aleksandra Mir called The How Not To Cookbook (to 4 Oct), a hymn to culinary disaster. As its name suggests, The Enlightenments purports to engages with ideas thrown up by the 18th-century Edinburgh Enlightenment, although I must say I cannot for the life of me see how.

Anyway, some of the works in the show are extraordinarily good nonetheless, notably Tacita Dean's Presentation Sisters – a typically Dean-like study of the last five members of a religious order in Cork, their rituals and impending doom the ideal subject for the artist's meticulous, melancholy eye. The animated clay figures in Joshua Mosley's lovely film, dread, include Jean-Jacques Rousseau and Blaise Pascal, which I suppose gives the work a certain Enlightenment cred. (Is Rousseau killed by a giant clay dog at the end, or merely licked a lot? You tell me.) On the other hand, Nathan Coley's Beloved – a copse of green-and-white painted spruce – seems pretty unEnlightened, not to mention pointless.

And the highlights of this year's art festival(s)? Apart from Eva Hesse, of whom more next week, a brace of American veterans. Agnes Martin spent many of her 92 years painting more or less the same thing, namely bands of limed colour on canvases six feet by six. (At the age of 83 and weakening, she reduced them to five by five.) Eight of these are on show at the Dean Gallery (to 8 Nov), and the quiet power they exude suggests why Martin was loath to change the way she worked. At Inverleith House in the Botanic Garden (to 11 Oct), John McCracken's shiny resin stelae are likewise inexplicably moving – colour made three-dimensional, Ellsworth Kelly goes to Easter Island. Like Martin's canvases, the 75-year-old McCracken's monoliths are effortlessly powerful, taking over the space they inhabit and shaping it as their own.

Of course, the real point of the Edinburgh Festival is arguably its off-piste shows, and high on the list this year is Rose Frain's Alexandria Light (Miracles Gallery, to 29 Aug) A Wunderkammer of found objects made relics, Frain's installation muses on the messy history of Alexandria from wonder of the world to Third World. How did the city get from Antony and Cleopatra to its current state, and what does that change portend? Frain is too clever to give any answers, but she does provide us with clues, some narrative, some formal. Make of these what you will, as you're supposed to.