ABOUT optical illusions etc. We animals survive through knowing our way around the world. It's always impressive the way cats, say, refuse to look in mirrors because they realise that mirrors are just hopelessly confusing sources of information. The love of getting lost, the pursuit of bafflement, are distinctly human things. But if that's the kind of creature you are, proceed at once to the Hayward Gallery to see the work of Anish Kapoor.
For the past 15 years this sculptor has been successfully undermining the known world. His first pieces were regular and irregular objects covered in coloured pigments so intense that the eye trying to register their solid volumes only saw a shimmering shape. Later, he began sinking dark cavities into rocks, such black holes that there seemed to be literally nothing inside. And in the past few years Kapoor has brought his illusionistic skills to what looks like their destined perfection.
This recent work is what fills the Hayward. Some of it is sunk into the walls or floors. Some of it is enormous. It's heartening to see an artist knowing completely what he's doing, and I doubt if anyone could not be staggered by these sundry apparitions, transformations and vanishing tricks, whatever value they might put on them afterwards.
The first piece you meet gives a pretty good idea of what's to come. It's called My Body, Your Body and it looks at first like something definite: an oblong, the size of a door, painted in deep blue on the white wall. But then you know how in cartoons the prankster will sometimes "pick up" a man-hole - take it from the ground as if it were a black disc and lay it down again in the path of the fall-guy, whereupon it turns back into a hole? Here it is done for real.
For, approached, this flat suddenly gets deep. It's not painted on, its an oblong slot cut out of the wall, with a deep blue cavity behind. You can put your hand through it. You can catch an echo of your voice in it. But look in, and another thing happens. This cavity appears, not as a hollow chamber, but as a boundless void. You know there is a surface in there somewhere, and not too far back. You know you're looking straight at it. But the eye can't find a place to rest. All you can see is a deep blue yonder, a space without coordinates. The eye's first guess was that this floating blueness was painted on the wall. Now it seems to be nowhere at all.
And then a further development: you can, at certain angles, just get a glimpse of what's really there, the interior contour of the cavity. The eye catches a curved highlight, revealing the rim of a plunging, vulva- like hole. But this form is visible so fleetingly that you never get a clear view of it, and may wonder whether it, too, isn't an apparition. The only constants are the oblong shape and the colour blue.
I dwell on My Body, Your Body because it has all Kapoor's main effects - an illusion of something definite, an illusion of something absolutely indefinite, a glimpse of what's really there. In other works they happen in a different order, or with different emphases. The results can be even more astonishing.
Untitled, which sits in the second room, looks like a huge, curvaceous, futuristic telly with a wide screen showing luminous grey. But this "screen" is another slot, through to another hollow, another void, and the whole thing becomes a kind of tardis, a finite fibreglass egg containing an infinite gulf of air - and you really can't focus at all on the inside of the cavity.
Or again, what's really there may be quite obvious illusion, when it happens, the more startling. Upstairs, there is another fibreglass object, White Dark, a 10ft-high white cuboid, with a great dimple-crater smoothly sucked into one of its faces. You see that clear enough. But move a little, and this face goes firm and flat like the rest. Move again and the solid form dematerialises, turns to a vague mist. When I Am Pregnant is more daring still. A teet-like protuberance, rising out of a wall, though sharply visible in profile, simply disappears when seen straight on.
All this is very satisfying. The art is sure of its effects. They are perfectly judged and perfectly executed. (I wasn't so interested in the works that used mirrors; their surprises weren't so surprising). What's more, this sureness promotes community among viewers. We're all having the same disorienting experiences and we know it. It's not a matter of taste or imagination, it's a matter of eye-brain function, and this unanimity, rare in art galleries, is satisfying, too (though a little frustrating for the critic, who can only add his coo to the chorus).
But beyond this basic shared experience, Kapoor's audience is likely to divide significantly. Standing, say, beneath the final work, At the Edge of the World, a massive red bell-dome which, when you get under it and look up, acquires the elevating vastness of a starless night sky, one lot of viewers will be saying: "I am losing all sense of the boundaries of the world and of myself, I am entering an altered state." And these people will probably be more susceptible, also, to the many ways that the experience can be interpreted - psychically, sexually, spiritually - indeed, there really seems no limit to the metaphors you can make of Kapoor's illusions. But another lot of viewers will simply be saying: "This effect is extraordinarily clever and fascinating, I wonder how it's done."
You could feel both, I guess. I had a really spacey time of it, I'm with the latter lot. Bewildered, I want to know how I'm being bewildered. I mean, Kapoors lighting must be very carefully controlled, but are specifically light-diffusing pigments and materials being used? And the smoothness of the curves is clearly crucial to making contours dematerialise, but is some actual maths involved too?
And this practical attitude is likely to take Kapoor's superbly calculated disorientations less as a revelation or an ecstatic state or even a thrilling semi-scare, more as a salutary warning of how fragile our sensory survival mechanism can be. What some see as sublime, uncanny, are just breakdowns and short-circuits. If all life was like this, we wouldn't stand a chance. The bottom line is, we have to sort such things out. Or that's what my lot says.
But what I think is admirable about Kapoor's work is that it doesn't insist either way. It allows each party its response. It holds its ground between the material and the transcendental. It urges only, what is plainly true, that things can sometimes get very mysterious, and mystery always makes a claim. Humans clearly have a penchant for such breakdown experiences, and perhaps are bound to find them meaningful. A cat would sensibly avert its gaze, but we can't tear our eyes away.
Anish Kapoor's work is at the Hayward Gallery, London SE1, until 14 June.