Ian Hamilton Finlay is a poet and philosopher and one of Scotland's most successful living artists; yet, like all prophets worth the name, he has never found much favour in his own land. Now, thanks to the rejuvenated Serpentine Gallery and the wisdom of the Royal Parks, London, at least, has a permanent example of his work. Edinburgh should take note.

The London "sculpture" consists of 8 benches, a tree-plaque and, at the door of the gallery, a large slate disc set in a field of cobbles and inscribed with the names of trees found in the gardens. Within this is a quotation from the 18th-century philosopher Francis Hutcheson, and, at the centre of the circle, a dedication to Diana Princess of Wales, the gallery's former patron.

Depending on your take on these things, this is either an utterly pretentious waste of good slate, or one of the most moving marriages of art and landscape that you will find in the capital. As you may have guessed, I subscribe to the latter view, though why is not something that I can easily explain. It has to be experienced to be understood.

The inscriptions carved on the benches are taken from such varied sources as Virgil and C Day Lewis and, read cumulatively, have a hypnotic quality. As certain key words occur from one to the next - smoke, shadows, roof- tops - the mood that these conjure is a very specific one: "a mixture of sadness and tranquillity" to borrow a phrase used to describe Virgil's Eclogues and quoted at the beginning of Hamilton Finlay's Proposal, a beautiful little book published to accompany the commission.

I'm aware that the bare description of all this sounds rather bald and, yes, not a little pretentious, but, in fact, quite the opposite is true of the thing itself. Despite the hum of traffic from Exhibition Road, Hamilton Finlay has turned this bit of the gardens into a place to pause and think. It is, indeed, tranquil and even a little sad, but with a gravity and understated elegance that seems entirely appropriate.

Ian Hamilton Finlay, a permanent commission in the grounds of the Serpentine Gallery, Kensington Gardens, W2 (0171-402 6075)