Naivety in art is a puzzling thing. There are unschooled artists, such as Henri Rousseau or Alfred Wallis, whose images we find strangely sophisticated. And there are sophisticated artists who carry on like ingénues.

The critic Christopher Ricks, writing about the poems of Stevie Smith, captured part of the puzzle. "Can she possibly be as ingenuous as she sounds?" he asked – and answered: "An ingénue is of interest only if you can't be entirely sure." In other words, if you can't be sure whether the naivety is true or false. The genuinely naive is too simple; the faux naïf is too knowing. But something between the two...

Take a near contemporary of Smith, Stanley Spencer. Is his art naive? It has some of the marks of naive painting – the wonky bodies, the wonky scale, the love of details for their own sake. Yet clearly that's not the whole story. Spencer knows all about Picasso and Giotto. He can paint in an un-naive, skilfully realistic manner when he wants to. So is he putting the naivety on?

Consider one of Spencer's strongest effects: the sense of close physical contact with which he imbues his scenes. Things are housed tightly within other things. Multitudes are clustered together. Everything is snug and hugger-mugger. It's a feeling straight from early childhood, a world of clambering, burrowing, snuggling.

It's also a source of awkwardness and strain. In these scenes, things end up in crushes, pile-ups, scrums and tangles, sheer jam-packedness. And does this awkwardness arise spontaneously, out of Spencer's childlike love of snugness? Or does he deliberately cultivate it, exaggerate it, as the sign of a "childlike vision"?

Convoy Arriving with the Wounded is the first of Spencer's wall paintings at Sandham Memorial Chapel. It recalls his own First World War work, as an orderly at Beaufort Hospital near Bristol. It shows a convoy of double-decker trucks coming up the drive.

Clusters of wounded men, a packed arrangement of white triangular slings, fill the upper decks. Rhododendron bushes burgeon, while the verticals of the red iron gates, and the shadows they cast, thicken the texture. But the advancing trucks are the most surprising example of snugness.

The lead vehicle is like a toy truck. Its forms are all squared off, as if it were made of simple woodwork. The front of the upper deck is perfectly flat, and perfectly flush with the radiator beneath. The "nose" of the engine has been compacted squatly, so as to flatten things more. It's actually more like a house, a doll's house.

And it proceeds along the approach road as if squeezing down a channel. The massed rhododendrons hug the truck's sides, brushing, pressing. The truck seems to be burrowing through them, or simply to be held there, firmly but gently. For many 20th-century people, the experience of being on the road meant freedom, unattachment. For Spencer, it has a womb-like containment.

The clincher is the way the truck makes its entrance – or rather doesn't. There would be two ways to suggest that the vehicle was travelling through this gateway. You could show it still approaching, just coming up to the threshold. Or you could show it having just crossed over the threshold. Spencer avoids both. He aligns the front of the truck exactly on the threshold. It fits snug in the tight entrance like a flat-fronted house in a terrace, a biscuit tin on a crowded shelf.

The only indication that the truck is in transit is the turn of its two front wheels, as it steers into the yard. The detail is sweet, it makes you smile. The implied manoeuvre is impossible. How would the vehicle extract itself from this tight spot? And it's the point where you have to wonder: does Spencer genuinely not notice the awkwardness of his scene – or is he consciously playing it up? Are you smiling at him, or with him? The beguiling thing is, you can't be entirely sure.

The artist

Stanley Spencer (1891-1959) never really grew up. He was born in Cookham, Berkshire, and lived most of his life in the village. Through art school, war service, two marriages, he retained the total self-centredness of an infant. His masterpiece is the Sandham Memorial Chapel, inspired by Giotto's Arena Chapel in Padua. He found holiness in everything around him, and planned a "Church of Me", a building decorated with images of his life. Late in his career, the British Council arranged a trip to China. When introduced to Chairman Mao, Spencer said to him: "As you know, I am from Cookham..."