A visit to the new show of Innes's work in Edinburgh would not necessarily have cleared up the confusion. The winner of last year's NatWest Painting Prize may paint on canvas and paper rather than on brick and plaster, but the pictures in Innes's show - seven oil paintings and eight watercolours - are all about work methods. That is to say, their subject is the process of their own making: the way in which their creator mixes his paints and handles his brushes, the artfulness with which he applies a turpentine glaze or water wash. Innes may be known as a geometric abstractionist, but his work is only partly about the counterpoise of shape and form. It is also narrative painting, constantly acting out the drama of its own creation.
Take any of the oils called Exposed Painting: Charcoal Grey. These seem easy enough: a white ground on which Innes seems to have painted two rectangles - one of a strongly glazed black and the other a thin, matt stone colour - that overlap to form an L-shape. Look at your chosen picture again, though, and something seems wrong. The drama of the painting lies in its overlap. Your eye tells you that a thin, light wash applied over a very much darker and glossier one could not possibly produce the result you see before you, and it is right. Innes has painted a black field and then painstakingly removed half of it, dabbing away with turpentine to leave a pale rectangle of paint residue. Just to let you know that all of this is going on, the precise edges of the rectangles blur at the moment of overlap, the thinned-down oils bleeding out to trickle to the edges of the canvas. To trompe the oeil a little further, the black field on which all this gentle havoc is wrought is not black at all, but a confection of greys and red or yellow oxides.
Now you might say: so what? Why should Innes's wrestlings with his medium be of any interest to us? The answer, I think, is that these are elemental paintings, ones that reduce the act of looking at art to its simplest forms. They are, pace the Edinburgh rating authorities, about the bricks and mortar of painting, and Innes's visible tussles with his pigment suggest the workmanlike nature of his task. I'm not convinced by the apparently happy accidents in Innes's pictures, the turps that bleeds just so, the pigment that runs, ever so sculpturally, over the edges of his canvases: but if he's cheating, the power of his work is still beyond question.
There are other reasons for travelling to Edinburgh to look at Innes's show, the first being that you can see where he is coming from in terms of local inspiration. Walk around and you will see Inneses everywhere, in the biscuit-and-soot stone of Edinburgh's buildings. Walk as far as Inverleith House and you will also see where he is coming from in art historical terms. Agnes Martin, 87 this year and the mother of all geometric abstractionists, is showing a dozen new works there, as well as a video of her 1976 film, Gabriel.
For anyone who has found Martin's straight-white-line aesthetic impenetrable, this film will provide a handy revelation. What it suggests is the importance of landscape in her work - not the figurative reproduction of landscape, but the re-creation of the means by which that landscape is made. Like Innes's, Martin's paintings are about the process of adding and removing, the pencil lines that are left on her apparently meticulous canvases, the cardamom red that leaches out from beneath a thin white wash. After the sturdiness of Innes's work, these pictures seem almost painfully refined: they talk in whispers. What they have to say is unexpectedly similar, though. We're lucky to be able to see these two artists together.
'Callum Innes': Ingleby Gallery, Edinburgh (0131 556 4441) to 11 September. 'Agnes Martin: New Paintings 1995-1999': Inverleith House, Edinburgh (0131 248 2943) to 31 OctoberReuse content