Dan Flavin

I'd never seen a room blush before I stepped into the tall drum of the Serpentine's central gallery, its walls rigged with Dan Flavin's 'Monuments for Vladimir Tatlin' (1964-75). But then it happened. The chilly white surfaces flushed lilac, pink and then - as if remembering themselves - faded to a sombre grey. Maybe it was the visitors' fault. Flavin's light works are as affecting as a weirdly beautiful person standing at a bus stop. Faced with such loveliness, it's pretty hard not to spin meaning around it, investing it with special powers and high hopes. But we don't know the boy waiting for the number 38, and the compliments racing round our heads reflect nothing more than our own peculiar yearnings. It's an impulse Flavin tried to counter in his audience, insisting his work was 'what it is, and it ain't nothing else'. These neon armatures don't contain some fuzzy spiritual truth. They don't, as one reviewer in a 1967 issue of Art News insisted, 'burn out, desiring a god'. The bare strip-lights are just bare strip-lights. No wonder the gallery blushes.

If Flavin expects us to be good Modernists fixated on the ontology of his light fittings, his titles don't make it easy. Monument 4 for those who have been killed in ambush (to PK, who reminded me about death) (1966) is a case in point. Four red neon bars in a dark corner, its searing lines are laser beams lashed to heavy, makeshift splints. Its reflection appearing on the gallery floor like bad news on a radar screen, Monument 4 ... is an uneasy war memorial, abandoning glory for the trapped-rat terror of a night attack. Once you get to thinking about teenage GIs scrambling around the Vietnam jungle, a purely Formalist response starts to feel a bit inadequate. Maybe we're on safer ground with Untitled (to Henri Matisse) (1964). A vertical rank of blue, green, pink and yellow bulbs, the DIY store colour-ways bleed into each other, a white-light counterpart to the brown mess of a paint box overloaded with mixed-up acrylics. While the piece makes a neat point about the capabilities of Flavin's materials, it's difficult not to wonder whether this bleaching of Matisse's Mediterranean palette has something to say about the geography of Modernism. If the pre-War avant-garde was characterized by the seafront pastorals of Picasso's Antibes and Matisse's Nice, Flavin's albino neon offers an industrial urban glow, unconcerned with the vagaries of natural light.

The installation of Untitled (to you, Heiner, with admiration and affection) (1973) playfully introduces Flavin's art to the natural world, its luminous green grids echoed in the verdant squares of grass visible through the Serpentine's windows. Stretching across the gallery space like a snaking pack of cards, the fluorescent fittings create a low wall between the viewer and the windowpanes. Stay in the room for a few moments and the parkland beyond the glass turns pink. Behind its double barrier, nature looks as synthetic as bubble gum, while the interior hovers uneasily between an emerald forest and the cartoonish laboratory of an evil scientist. Of course, the pink grass is a trick of the light, an afterglow of complementary colour following the dominant neon green. But, twinned with this bank of windows, the seeping illuminations of Untitled suggest larger tensions between natural and manufactured environments. A gallery marooned in Kensington Park (a swatch of greenery hemmed in by choked west London streets), the Serpentine has always riffed on the theme of art versus nature. By setting Flavin's strip-lights against a more complex backdrop than their usual four white walls, the curators have drawn something unexpected from an artist overgrown with brambly 1970s theory.

Saul Bellow speaks in interviews about his 'ideal reader', the bookworm who responds to every flick of the switch. It's an engaging fantasy for an author, but pretty prescriptive too. Flavin has called his art 'psychologically indifferent decoration', providing 'a neutral pleasure of seeing known to everyone'. In material terms alone, there's a lot to get excited about in Flavin's neon tubes. His gorgeous objects bathe the gallery walls in darkening shades of honey and their reflections, like wobbly underwater swimmers, are distorted by the shiny floors. In the end, though, light is too deeply embedded in our metaphorical thinking patterns, too wrapped up with God and science and sunsets to let us approach Flavin's work with the neutrality he solicits. But if the Serpentine show fails to make us his ideal viewers, it connects his bulbs to a whole new power grid.