In the end, with Paul Nash's art, the question is: do you believe? I'm not saying that you can only enjoy it, if you do. Most of his fans don't. The artist himself possibly didn't. He was first and last an artist. Still, the work is so spellbinding, it raises a question of belief. It goes beyond symbolism, beyond a theatrical shiver. It asks you, quite seriously: do you believe in ghosts?
Ghosts. All right, it's not the right word. I don't mean something white and flitting, or an armoured man with his head held under his arm. The presences in Nash-world are something far less defined and less definable. It is haunted all through. Or that's partly it. But I'm not sure that even Nash found the right words for his spell.
In his essay "The Life of the Inanimate Object", he wrote about "the endowment of natural objects, organic but not human, with powers or personal influences..." The hills are alive – and the rocks and stones and trees are too! Yes, with any English outdoor art, especially one that is set in a handful of favourite locations, these Wordsworthian terms are going to be tempting.
But go to the show that opens in Dulwich Picture Gallery on Wednesday – Paul Nash: the Elements. The powers that dwell in these landscapes don't feel like quasi-persons. Nature doesn't wear a human face. When Nash's work is at strength, it's as if another and quite alien world had intersected with this one; as if the hills etc had been taken away and then returned, subtly changed.
Nash belonged to the very diverse generation of British modern artists who went to the Slade and included Wyndham Lewis, Stanley Spencer, Ben Nicholson, Dora Carrington. But you might wonder how far Nash needed to learn anything. Take almost the first work in this show, a drawing called The Pyramids in the Sea, done in 1912, when he was 23. Two pyramids, by moonlight, emerge from a turbulent sea. It's all there already. It's one of those mystifying early works that seem to hold a whole career in embryo, and to anticipate later influences.
Years before there was such a thing as Surrealism, this image finds a power in deep strangeness. Years before Nash had seen a Samuel Palmer (who only resurfaced in the 1920s) this image has a Samuel Palmer moon and a Samuel Palmer sea – that is, if Palmer had ever done a stormy sea. The waves rise in massive bell curves. And in those waves, Nash first finds the swelling-dipping forms that will provide him with hills for the rest of his work.
In a more agitated way, these same curves make the churned-up trench-scape in his First World War horror-masterpiece, We Are Making a New World. They are still there, the background hills, in the late mad vision of Solstice of the Sunflower, from 1945, the year before his death.
Nash has more than one mood, of course. He can do pastoral peace. Or in the cold views along Dymchurch shore there is an M R James spookiness. Here, there are figures (rare in Nash) and they might be drifting ghosts. But then you come up against The End of the Steps, with its cuboid block of solid concrete. It's a blank stop in the picture. It comes from somewhere else. It's inexplicable, immemorial, perhaps extraterrestrial. That is the essential Nash effect.
When Surrealism arrived, Nash consciously engaged with it. It's not surprising that he felt affinities. For a time, he was considered "English Surrealist-in-Chief". But his most orthodoxly surreal pictures are his weakest. Pictures like Northern Adventure and Landscape from a Dream are deliberate mix-ups. He is trying hard to be irrational.
It's a question of what value you put on strangeness. Party-line Surrealism played in negatively, a dissonance to stir up the mind, break down categories. But for Nash, when he's on form, the incongruous is only a step towards a mysterious synthesis, a way of conjuring other dimensions of experience.
Out of Surrealism, Nash created his few actual visions of the beyond. They're believable because you can't get your head round them. The extraordinary Mansions of the Dead shows giant shelving units and frameworks set in the clouds with flying discs gliding around them. What kind of space is this? Is its genre paradise, or sci-fi?
But his strongest scenes are set on Planet Earth. His trick is the confrontation with the incomprehensible. In Pillar and Moon, you notice the alignment between the stone ball finial on the top of the pillar, and the full moon in the sky: two globes, the same size, set almost at the same level in the picture. You can't say whether this echo is meaningful or meaningless.
Circle of the Monoliths has a field of Avebury stones, but each one apparently has its own patterned chair cover. This is bonkers beyond Surrealism, so much so that it seems to know something. But what? Or there's Event on the Downs, probably his finest work, all the more strange for being (so to speak) entirely in prose. Front of stage: a clump of tree-trunk and a tennis ball appear before us, side by side. Behind them the downs roll away. The "Event" is presumably their being together. What possible link between them? What brought them here?
Nash paints the presence that doesn't make sense. That why, rather than talking about animism or pantheism, which always have a humanising touch, it may be better to think of UFOs, just so long as it's understood that aliens really are alien. A good comparison is cinematic. Think of the moment early on in 2001: a Space Odyssey when the monkeys are startled by the arrival of the monolith, standing there, out of nowhere.
His Second World War masterpiece, Totes Meer, has the same effect. The graveyard of downed German aircraft, a vast sea of broken wings and fusillages is one of those sights that make you suddenly feel you have no idea where you are. The land been taken over, changed beyond recognition. Wartime landscapes must have offered such experiences quite often.
Even Landscape of the Vernal Equinox, wartime too but very different in content, a view of Wittenham Clumps, one of Nash's home subjects, has an unhomely feeling. With both a moon and a sun in the sky, it is a "kaleidoscopic" scene, which holds several spaces and lights, which the viewer can't resolve. You can only chop and change between them. It's a landscape with no certain ground.
People think Nash is a rather cuddly painter. He's a bit modern, yes, but safely English romantic visionary, who loves the land and fills it with character. Actually, most of his work goes the other way. As his photographs of lone stones, abandoned structures and smashed trees show – there's a good selection here – his inclination isn't to animate the inanimate. It's to make it even more inanimate, and resistant to our natural impulse to invest it with our life. Nature becomes as if landed from Saturn.
So what would you believe, if you believed what these pictures are saying? Something like: that the world around us is not ours. It belongs to realms that are beyond us. Nash gives us the kind of feeling that crop circles gave us, when they first appeared and their status was still obscure and unaccountable. It's the kind of feeling you still can get, momentarily, when you take a corner and the landscape lies ahead transfigured by miles of wind farm. We live in a changeling world.
Paul Nash: the Elements at Dulwich Picture Gallery, Gallery Rd, London SE21 (020 8693 5254) 10 February to 9 May, £9, includes permanent collection