I’m always fascinated by the tendency of abstract artists towards radical simplification. Not content with eliminating representational subject matter from their pictures, they go on to eliminate everything else, too, so that for most of their careers – several decades, anyway – they dedicate themselves to presenting the same simple idea over and over again.

One of the earliest of them, the Dutchman Piet Mondrian, reduced his pictures to grids of black on a white ground, with some of the resulting squares variously coloured. In England Ben Nicholson got rid of landscape or still life elements and painted a long succession of grids solely in white, sometimes very large, sometimes partially in relief. The American Abstract Expressionists of the 1950s opted for enormous canvases devoted to a single pictorial idea: Sam Francis made apparently random watercolour-like splashes, Jackson Pollock overlapping strings of thrown paint, Clyfford Still tall, ragged rents of colour in expanses of darkness. (As an aside, can I whisper that Picasso never painted an abstract picture in his entire career.)

Mark Rothko opted for the most minimal of all these procedures. He too favoured great size – critics at the time compared his preoccupation with vast scale with the nineteenth-century artists’ love of huge landscapes, the Rocky Mountains, Niagara Falls and so on. The word ‘Sublime’ was much employed. He reduced the content of his pictures to enormous squares of colour hovering on top of each other, sometimes strongly contrasted, sometimes, as here, subtly differentiated so that the eye hardly knows where one ‘colour field’ ends and another begins.