Roni Horn, Tate Modern, London

Big ideas create little of interest

Tate Modern is currently hosting two exhibitions on its fourth floor. At the eastern end, film clips show us scenes of turbulence – the flourish of bayonets, the teeming of people. These raw images are announcing the revolutionary ferment of Russian Constructivism.

Walk to the other end, and it feels, by contrast, as if you are entering some calm, sequestered, almost monastic place. This is a mid-career survey of work by the American artist Roni Horn. Most of it is quiet, understated, cerebral and, it has to be said, rather dull. What is the problem here? Horn is too captivated by the thoughts that provoked her work into being in the first place. She writes about these in the captions that accompany many pieces. The captions are probing, interesting and always well expressed. The work itself often feels like a pale afterthought.

Take her drawings, of which there are many scattered throughout the show. Generally speaking, Horn is not interested in making drawings of particular objects, but in the idea of drawing as a mode of thinking two-dimensionally, a way of charting her mental processes. Her lines are always going for a walk, in the famous phrase of Paul Klee. If they knew their destination, they probably wouldn't have set out in the first place. These are drawings that seem to be exploring the nature of what it is to draw.

They usually consist of two kinds of marks. One, in colour, looks as if it is moving towards some kind of ghostly, stuttery representation of some part of the human form. But then, when we get closer, we find that these coloured marks have been embedded in a kind of grid system of pale pencil marks, suggestive of something more schematic and architectural. This description makes them sound much more interesting than they look.

In the third gallery, a solid copper cone lies on its side on the floor. You wonder about it – then pass on. In the next gallery, another copper cone, seemingly identical to the first, is lying on its side on the floor. Why two? What excites the artist – she tells us so herself – is the nature of memory, how it plays its tricks upon us. Is the first cone identical to the second? Can we even hope to remember each one in sufficient detail for us to be sure that the first cone is or is not identical to the second? That is the conundrum that never ceases to fascinate Horn – the slippery nature of memory; and the equally slippery nature of reality, and how what we believe we remember may not in fact be quite what we have seen or experienced at all.

Well, this is undeniably true – reality may indeed change from one minute to the next. Your reality, like your crime scene, may be different from mine. But the fact is, these cones are not terribly interesting, except in so far as they provoke the thoughts described.

The exhibition does have moments of genuine visual allure. A sequence of photographs of the Thames almost fills – glug – one gallery, which appropriately overlooks the river itself. Under exploration here is the nature of water and how it lacks a stable nature, and is forever changing. We never stare at – or step into – the same river twice. The photos are an arresting documentation of this. As you look from one to another, you notice that, at a certain moment, by a certain light, the Thames is pocked, at another speckled. Then it seems covered by a thin film of plastic. Each photo has an extended caption, with numbered points. The artist lays out for us, in exhausting detail, what she thinks about water. Here is the 29th point she makes about one photo: "What are you thinking about? Are you paying attention to the numbers? Maybe you won't read all of these numbers." How true.

To 25 May (020-7887 8888)