Barbara Hepworth's bronze statue Two Forms (Divided Circle), has been stolen from a London park. It's thought that it hasn't been stolen to order by an art-loving millionaire for a garden ornament. It's been nicked by thieves aiming to turn a quick profit by selling it for scrap.

The price of metals has risen sharply in recent years. Bronze is a copper alloy and, according to the scrapyard I spoke to, gets £2.50 a kilo. The statue is 2.3 x 2.3 x 0.5 m – I don't know how much it weighs, since art curators don't usually reckon statuary like that. Perhaps they should start. Anyway, you can probably take a guess how much the thieves are going to get for it.

There are six other copies of Two Forms (Divided Circle), so it's not lost for ever to the world. You can see it in St Ives, Cambridge, or Evanston, Illinois. But there aren't so many that we can spare one or two, and this one is lost to Dulwich Park permanently. When I heard the news, I immediately thought "Christ – London's full of hideous metal sculptures. They could have taken that horrible one of the giant halfwits embracing at St Pancras. They could have removed the Memorial to the Women At War that looks like the cloakroom at the world's worst party. They could have taken away that Donkey Emerging Into The Sunlight on Park Lane. Why the hell did they have to take away my Barbara Hepworth?"

I love Barbara Hepworth. I first glimpsed her work in an introduction to modern art for children – it was just a polished wooden curve with some strings, as if pulling the thing together. It was just love at first sight, and I wasn't happy until I'd seen them in the flesh. Her forms went straight into my soul, and stayed there. You can't explain, always, why you love what you love, and it's like that with her. To look at them is to feel that you are holding them with your eyes. The magical garden of her house at St Ives – tropical, merry, and warm even when it's pouring with rain – is the most perfect artist's site I know.

What are they worth? There are a number of answers to this awful question. One is the thieves' – £2.50 a kilo. There is another, which is the collector's and connoisseur's, which is basically "as much as someone is prepared to pay for it". It's insured for half a million. In the great wave of accountancy which swept the nation in the 1980s, public authorities were obliged to tot up their assets, and work out what they owned, and how much it was all worth. Someone, I believe, had to decide how much Palmerston's Italianate Foreign Office would be worth if Foxton's flogged it. I would tell you how much a Hepworth goes for. But the sites which tell you the auction value of individual artists all require subscription, and I'm not really that interested. The work of art has a price; and, more than that, the price has a price, apparently.

The material price is all we ever seem to talk about here, whether we are approaching it from the point of view of a scrap dealer or a connoisseur of art. But there are other ways of thinking about it. It occurred to me that what I love most about the Two Forms (Divided Circle) is not the bronze at all, but the holes. Other sculptors have specialised in holes, but nothing is as radiant as the air that fills the gaps in a Hepworth – here, two shining holes and a vertical empty line, like light joining the heavens and the earth. And all around the Hepworth, a shining aura. It gathers up the air around it, and makes it blaze with energy.

You can value the bronze in a Hepworth: you can weigh it up and cost it, and melt it and turn it into widgets. But how are you going to price up the holes, the gap, the shining aura? Those things are worth something, supremely. It is obvious, when you see the sculpture. There is no price to be placed on it. Could the auction house put a price on those lovely lacunae? I don't think so. It is just a gift from the sculptor to us, not worth anything. It just makes our souls sing, that's all.

Art, in the end, is more than a copper alloy on a base, just as a poem is more than ink impressed on to paper, and a beautiful chair is more than wood and cloth. To put a price on it is to humiliate it, whether in assessment of the value of its substance, or costing up its aesthetic value.

One of my favourite museums in the world is the Folk Crafts Museum in Tokyo, where thousands of ordinary objects from anonymous craftsmen are collected – not because they are valuable, but because they are simple, everyday, beautiful objects.

The insurers and the thieves will be able to work out what the value of Hepworth's bronze may be. But the holes, and the gaps, the arranged air, the beauty that has no cost and no price – everything beyond the grasp of money is what matters.