In the mid 1940s when the Queen Mother purchased Paul Nash’s Landscape of the Vernal Equinox (1943) Princess Margaret remembers saying, “Poor Mummy’s gone mad. Look what she’s brought back.” But though this painting is one of the undoubted masterpieces of 20th-century British art, it’s easy to see why the Princess responded as she did. At a glance, the dry, scrubby brushwork, muted colours and somewhat lumpen forms don’t exactly sing out of the grandeur of the English countryside in the way that, say, the paintings of Samuel Palmer or John Constable do.

Arshile Gorky found it almost impossible to finish a painting. Something would always call him back. So he would go back and would add and retouch and tinker around over several years - sometimes over the course of a decade or two. “When something is finished,” he once said, “that means it’s dead, doesn’t it? I never finish a painting, I just stop working on it for a while. The thing to do is... never finish a painting.”

Modernist art movements are a lot like totalitarian regimes. They produce their declaratory manifestos, send forth their declamatory edicts, and, before you know it, a Year Zero mentality prevails: the past must be declared null and void. Seeking to overturn 1,000 years of Western civilisation with a universal aesthetic utopia of brightly coloured squares and boldly delineated lines, a confident Theo van Doesburg, founding member and chief theorist of the Dutch movement De Stijl, wrote, “What the Cross represented to the early Christians, the square represents to us all.

Dazzling and surprising, this Tate Britain retrospective by the 1998 Turner Prizewinner Chris Ofili should erase memories of the media sniping about him making money from using the so-called "gimmick" of incorporating elephant turds in his paintings. It will also confirm his status as one of the greatest contemporary British artists.

This exhibition may claim to reveal the real Van Gogh through his letters, but what of the Sunflowers, the Self-Portrait With Bandaged Ear, oh, and Starry Night, with its roiling night sky and dark, mysterious cypress tree? What even of the dizzying Night Café, with its migraine-inducing electric lamps, its violent clash of reds and greens and the walls that threaten to collapse inwards, as if the painter had been hitting the absinthe all night?

When we look at still images of moving figures what we see is not exclusively determined by what is in front of our eyes but what we already know about the world. If we stopped to think about this, it would seem obvious. We would know, for instance, that the putti who are so joyously leaping, dancing and bounding about in Donatello’s static frieze Cantoria would make little sense to us if we didn’t already know what such static postures implied: still images of moving figures can only come alive in the imagination when we have some understanding of how living bodies move, and of what comes before and after.

The latest official royal portrait, and the first painted portrait featuring the Princes William and Harry, hangs in a small room at the National Portrait Gallery among a selection of royal portraits of the Windsors. There’s the rather quirky one of the Queen Mother, painted in 1989 by Alison Watt, an artist who sought to capture her sitter “as ordinary as possible”. What our attention seems most drawn to is the china cup turned upside down on the arm of the Queen Mother’s armchair. Eh?