The hill was yellow now. But if it stayed yellow, Ken Kiff said, it might not stay a hill; and if it stayed a hill, it might not stay yellow. The remark wasn't only a description of his painting methods. It described what his paintings look like. Everything in them is fluid and metamorphic. There are figures, but there's no fixed scale and no visual realism. You can easily see that a hill might become a head, a hand might mutate into a tree root, and anything might change colour, because it already has changed colour dramatically. This painted world is imagination's plasticine.
Few paintings are at first sight as off-putting as Kiff's. What you see, or think you see, can be hastily diagnosed. It's the work of someone in therapy, a psychodrama, peopled by a small cast of archetypal elements, drawn in a rough, childlike hand. There's Man (often a little homunculus) and Woman (often dominating and sensual), and Tree and Flower and Hill and Island and various Beasties, and Cloud and Sun, with a face, and rays like petals or tentacles.
Sometimes more nightmarish elements intrude, or more everyday objects (a red letterbox, say), but the world remains a dreamland scenes from a psychic "journey". There's nothing but these symbolic fixtures, in a cramped landscape-theatre. Faces and figures signal primary, clown-school emotions – joy, fear, bewilderment, loneliness, aggression. And to keep the mood strong and simple-minded, the colours are indiscreet, radiant, lurid.
Kiff died in 2001, in his mid-sixties. At that time, it would have been hard to imagine a less fashionable kind of painting. If a contemporary artist had done anything like it – several did, actually – it would have been as a joke, an ironic imitation of naive sincerity. But things are now so up in the air in the world of painters, I'm not sure. Some play safe, holding on to a formula. Others, painfully or refreshingly, learn it all again from scratch. Possibilities are open. Kiff himself could be one of them.
For the time being, the only place you're likely to see his work is in the ultra-conservative Marlborough Gallery (the late artist's dealer). They're showing a "mini retrospective" – a random selection of works which happen to be unsold.
You can often guess why. The exhibition doesn't display him at his strongest. The most recent paintings – Iris, Tree and Sun, Person Writing – are the best. But Kiff is an artist worth not forgetting, so I take this chance to wave in his direction.
The trick is not to focus on the psychodrama, or at least to take it as one ingredient in a form whose great virtue is its free way with all its ingredients. Subject-matter is something, but the important thing is the way it is in open interchange with shape and colour.
Kiff's painting has extreme limitations. It's quite uninterested in the visible world, or anything to do with looking. It's concerned with shapes and the shaping action of the hand, and how one shape can modulate into another, acquire or change an identity, gain limb, body, face, expression, then lose it again (as when you knead all the bits of plasticine together and smear them over). This process of amoebic transformation, and a similarly busy mutability of colour, is where the action is.
Or rather, the point is not this figurative flux. It's how Kiff manages his evolutions and transitions, with soft fades, hard cuts, wrenching, strenuous labour, wild explosions, and graceful clumsiness. I regret that it all takes place in a Jungian Magic Roundabout. I don't see why the colouring needs to be so uniformly major key. And I squirm at the cuteness of Kiff's figures (the little bodies with big heads, the silly faces, the friendly cauliflower trees). OK, some of this is meant to be funny – an out-of-control "sorcerer's apprentice" world – and sometimes it is funny. Metamorphosis is farce, as well as affliction and liberation. Kiff is a true Ovidian talent.
Peter Doig's career has been one of the most encouraging facts of contemporary art. In an art world of instant yield and paraphrase, Doig's painting has prospered without it ever becoming plain what it's "about".
Doig's mid-life retrospective at Tate Britain is terrific, but at the end of it the issue is no clearer. The pictures are mostly landscapes: snowscape, prairie, forest, jungle, swamp, with a figure or a building. They are richly absorbing, continually surprising, and irreducible. They have what Delacroix called "the first merit of painting, to be a feast for the eye".
People find ways of talking about them. They call them disturbing, unsettling, full of menace. In the fizzing colours and congealing textures, they see signs of toxic pollution, or druggy delirium. They stress how his images are often derived from photos – so they're all a fiction. Or, in a more old-fashioned vein, they emphasise the abstract painted surface.
All these responses are right, in some degree. But that's a mark of how large Doig's paintings are, in size and scope. And though some form groups – the Concrete Cabin works, where modernist buildings are glimpsed through a dark forest – normally each painting is a new idea, perhaps of baffling unexpectedness. No art could be further from our standard model of artistic production, where a defined formula is repeated with minute variations.
What they're about is the stuff that landscape painting is often about: things being a long way away, out of range, obstructing other things, blending, forming patterns. Visual obstruction plays a large part: the world through branches, detritus on a pond. The paint, too, performs as an obstruction, making the image and getting in its way, adding flecks or taking off on a life of its own.
You can see Doig as a sort of post-Impressionist. But the gorgeous surface-pattern tendencies in his work are interrupted – by depth, narrative detail, or shifts of focus. He is always adding and connecting, as in the curious echoes he arranges between subject-matter and medium, where different things are rendered in the same kind of paint and colour, and you wonder why? And though you might suppose sheer information overload was the name of the game, the pictures choreograph their proliferations, and know where to stop. Recent work is sparer, but the ideas are still coming. This body of work offers an experience rare in current art: it feels big enough to live in.
Peter Doig: Tate Britain, Millbank, London SW1, to 27 April. Tate Britain is offering Independent readers the chance to book two tickets for the price of one, on tickets booked before 25 February. Call 020-7887 8888 and quote "Independent 2-for-1 offer". Booking fee applies