Rebecca Warren, Serpentine Gallery, London

Once upon a time, artists used their hands and eyes to create art. The brain alone is no substitute

Why is so much contemporary art so annoying? I refer you to Rebecca Warren, who describes her work like this: "Though [it] evolves through a process of appropriation and reference, it is non-didactic, being closer to revelation and discovery." And what does the one-time almost-Turner Prize winner mean by that?

Well. Warren does not make sculpture but "sculpture". At her art school – Goldsmiths, naturally – she will have received an education long on theory but short on practice. Fine arts were taught by a man best known for conceptualising a glass of water as an oak tree – a neat trick, although a little tired when compared with Duchamp's bottle driers. Any feeling for materials was assiduously bred out of Goldsmiths students. Art was of the mind, not the hand. The hand could be invoked, but only in quotation marks, to show how stupid hands were, how very clever minds.

And so to Warren's words, and to her show at the Serpentine Gallery. The work that greets you as you enter this is called Cube (2006), although it turns out to be cuboid rather than cubic. The sculpture (take the quotation marks as read) looks like clay, but is actually made of bronze. As deceptions go, these hardly rank with passing off a glass of water as a tree, but they do follow in the same tradition.

As well as misleading viewers as to its shape and medium, Warren's work also tricks us about its ancestry. Cube's monolithic form and name suggest descent from a tendency in modernist sculpture represented most memorably in Britain by Henry Moore. For all their newness, Moore and his pards bought into a belief about the relationship between the hand and the object that dates back to Plato. The block – the cube – has a totemic place in the history of sculpture, as being the thing from which Michelangelo freed the inevitable, the pre-existent, the divinely Platonic David.

So what Warren's lumpy artwork does is to define itself by affecting to be all the things it is not. Here I am, it says, a block, modernist, clay, a cube; and it is none of these. You might see the piece as anti-sculpture, or Bad Sculpture, although that would require it to engage with the traditions of art-making, if only to reject them. Actually, Warren's work is more revolutionary than that. It isn't Bad Sculpture or anti-sculpture but unsculpture, a thing given physical existence only to point up the valuelessness of physical existence. Cube is a thought whose expression required Warren to make a bronze lump on an MDF platform and wheels; although you feel she would much rather not have bothered, because the real point of her piece is the thinking behind it.

Now, conceptual art can be extraordinarily fine and moving, as witness Mark Wallinger's The Russian Linesman, a work-cum-show whose genius is its genius. The trouble is that conceptualism's medium – the thing it's made out of – is the mind. As Michelangelo proved, you can get good sculpture out of bad marble. You cannot carve good concepts from bad thinking. What do Rebecca Warren's elisions of Degas and R Crumb, her blobby-legged cephalopods and ho-hum vitrines tell us? That she has some knowledge of art history and rather more of art theory; that she believes that sculpture, in any traditional sense, is dead; that, in short, she went to Goldsmiths. To which I find myself saying, So what?

It's the vitrines that really bug me. For all her talk of revelation and discovery, Warren's Helmut Crumb – "iconic", apparently – is so clearly the output of an over-taught mind that it's hard even to be annoyed by it. Meret Oppenheim made teacups out of fur in 1936: does Helmut Crumb move us on from there? But Warren's vitrines seem to be trying to do something more heartfelt and less glib, to engage with our eyes and hearts rather than merely with our post-postmodern brains. And they do it so badly. Denied the easy solace of irony, they fumble about, managing to be neither elegant nor naive, mired on the road from St Ives to Hoxton, hinting at stories that fall apart as soon as you look at them. See this show if you're in the park and it's raining; otherwise, go rowing on the Serpentine.

To 19 Apr (020-7402 6075)