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The informative caption has become the distinguishing mark of the gallery. But if an artwork needs to be explained to be appreciated, asks Jonathan Myerson, doesn't that make it a failure?

At its best, the theatre rehearsal room is a fertile place, a democratic effort where anyone can suggest anything. It can also become a self-obsessed place, focusing too relentlessly on the hermetic world of the play. As a result, an actor, director or designer might suggest something that's just too many thought-jumps down the line or simply too unpredictable, something which will be incomprehensible to the audience. At such moments comes the sarcastic cry from the back of the room: "It's OK, we'll put a note in the programme."

At its best, the theatre rehearsal room is a fertile place, a democratic effort where anyone can suggest anything. It can also become a self-obsessed place, focusing too relentlessly on the hermetic world of the play. As a result, an actor, director or designer might suggest something that's just too many thought-jumps down the line or simply too unpredictable, something which will be incomprehensible to the audience. At such moments comes the sarcastic cry from the back of the room: "It's OK, we'll put a note in the programme."

This jokey rejoinder is the theatre's way of bringing itself back to earth, reminding itself of its primary task. By all means interpret the text, audiences tell us, take it anywhere you want, but everything must be carried by the two-hour traffic on the stage. You cannot ask the audience to bring any special interpretative powers to the production, any foreknowledge. They may have gobbets of information, they may even have seen the play or actors before, but that offers the cast no justification for the unexplained, unearned moment.

Now let's drive over Waterloo Bridge to Tate Modern. After its first flush of success, they've shuffled the collection and the Bourgeois spider is gone. Here instead is Mary Kelly's Post Partum Document: Analysed Markings... At its core, this comprises 10 sheets of coloured paper, each A4 page divided into three columns. The first column contains roughly typed, phonetically-spelt child's dialogue; the second column contains an all-caps sociological commentary on the child's behaviour; the third, hand-written poetic ramblings. Finally, someone seems to have childishly scribbled a crayon over these pages: red circles, blue zigzags, green mish-mash.

Maybe this does something for some of those standing there. But I doubt it does anything for anyone before they've read the panel that tells us how the artist recorded dialogues with her son, typed them out, commented them and then finally handed the pages back to her child to scribble over. Clearly, these off-stage facts are key to what this work is saying about the mother-child relationship, "the interplay of voices, the mother's experience". And yet where are these facts contained in the work of art?

Now look across to the other side of the room. Here's what look like promotional stills from a 1950s film. Photographs with the moody timbre of Hitchcock - the cloudy, guarded landscapes punctuated by a woman in tweedy fashions and permed hairstyle. I don't recognise the actress. Unsurprisingly, they're called Untitled Film Stills. But wait, they're not offcuts from a movie as campaign: the label tells me these are photographs of the artist herself, Cindy Sherman, posing in immaculately researched and recreated pastiches.

But how am I supposed to know any of that without reading the label? Impossible. I don't know Cindy Sherman from Eve; most of us wouldn't, why should we? How can I know that these works challenge "female roles in society", or challenge "the clichéd representation of women by the mass media"? Without the side-bar of information, these are just more Sunday supplement studio-system black-and-whites. Where's the statement, where's the art in that?

Now suppose you stop off at Waterstone's and pick up a new novel. You get home, ready to dive into this new, imagined world. But the story doesn't start on page one. No, before the prose gets underway, the novelist has an essay for you: he's going to tell you exactly what he's trying to achieve, what stereotypes he's hoping to demolish - because you certainly won't find any indications of this in the narrative. Then you start reading the story, but the author interrupts periodically, going through concepts you will never derive as you read his prose. Can you imagine enjoying such a novel? Do you arrive at a ballet, ready to be lectured on the ideas behind the choreographer's working methods? With the exception of TS Eliot, has any poet ever published notes to accompany and explain their verses?

So why do the visual arts so often insist on these Coles Notes to steer your path through their creations, indeed to explicate them at all? And does it matter if they do?

Yes, I think it does. Art, in any form - verbal, musical, visual - should be a primarily visceral experience. It's about responding instantly or in spite of yourself, losing yourself in the world created, it's about emotions. And every extra chunk of information you have to carry alongside, inviting your brain into this heart-and-soul process, inhibits that gut response. Every time you have to remind yourself that this face belongs to the artist herself, or that that's real elephant dung on that canvas, you interrupt the suspension of disbelief.

I'm not talking about abstract or non-figurative art here. You might look at Josef Beuys's Lightning With Stag in Its Glare and never decipher the quadrant blocks of silver-painted wood as the stag or the rusting steel hand trolley as a goat, but that is the failure of the symbolic artist. He lives or dies by that sword.

I'm talking about the art that is wholly reliant on the captioning to breathe. Go back downstairs and have a close look at Peter Fischli and David Weiss' installation. This is a classic type of modern art, the one where you can't decide whether it's art or a room they're half way through redecorating. According to the caption, this "appears to be a scene of work, full of tools and materials, as if a builder has just stepped out for a moment". And it does: trays of paint, mastic guns, an old toy dinosaur, a radio, a Lucozade bottle, pallets, all spattered with the dust and the fine white spray of a thousand previous decorating jobs.

But if you read on, you discover that all the objects are "carved from polyurethane foam". You look again. No, they can't be, you say. You wait for the attendant to be distracted and reach over the rope to lightly touch: and it's true, they are light, insubstantial, brilliantly perfect replicas.

But you could have stared at these objects for a lifetime and never realised. Indeed, you're not supposed to: "Fischer and Weiss have devoted considerable effort to hand-crafting objects that are usually made quickly and efficiently by machines." They must indeed have worked hard - though I was brought up to believe that art is supposed to be hard work - but what is the point? For me, the crucial flaw is that this deception can only thrive intellectually in the mind of the viewer, never viscerally. It's a tragedy of wasted energy.

And the tragedy doesn't end there. On the other side of Southwark Street, In the Jerwood Space, Catherine Yass is exhibiting some photographs. Giant slides, mounted in lightboxes, they are simple, exquisite, alarming, and, if it wasn't too much of a pun, illuminating. Portrait: Springfield apparently features six individuals associated with a psychiatric hospital. But it doesn't matter that you don't know that - the faces, in sharp focus, sit crisply forward, human and fleshly, while behind them, the backgrounds are blurred, making blue blobs of neon lights, brown clouds of industrial chimneys. You don't know whether these are patients, staff or doctors and it doesn't matter - they're hypnotically engaging portraits.

But alongside this sequence comes another, Metro: Prague - simple, blunt photographs of futuristic orange plastic walls and chrome rails. The gallery information tells me about this "shiny bright optimism in marked contrast to the grey weight of history above". Run that by me again: how can I possibly know what these walls are or, even if I buy wholeheartedly into the title, what Prague above now looks like? Maybe it has all been modernised in this 1970s-retro style. Yass is simply asking me to bring too much to the party. If you want to paint this contrast, paint it; don't demand that I merely imagine it.

Maybe this is my cultural upbringing, but as an audience member I want my first response to be emotional. Only later do I want to move on to admiration (as I continue to enjoy the work), and possibly (if I especially admire it) an interest in the methodology or artist's biography. Why has so much contemporary art reversed this process? it asks you to start with a dispassionate interest in the (often invisible) methods, move to dutiful admiration of the lengths the artist has gone to, and thus finally to emotionally respond to the art.

This flies in the face of what art is, the difference between art and journalism, between creation and reportage, and I just don't buy it. Art - any form - is art because it finds its truth by appealing to our emotions, not our intellect. I don't go to art galleries for essays, but for a heart-jolt.