Once upon a time, I genuinely believed that a poem was a sacred object, as unbudgeable and unchangeable as the great, leaning dolmens of Stonehenge. That was when I was very young.

Later, when I began to study poetry in earnest, I had the equally foolish notion that poems were probably written by language itself. Such was their incantatory authority that they seemed to have sprung into being without the active intervention of the human heart or the human hand. From an editorial point of view, it was a question of noli tangere.

What changed my mind in part was my own involvement in the world of book reviewing. One day I was sent the page proofs of a new collection of poems by Seamus Heaney. As I was looking through them, I saw, to my amazement, that someone – it was undeniably Heaney himself – had crossed out one word and substituted a better one in its stead. It was a moment of revelation. Poems could be fiddled with, even at proof stage. Things could be made better. The world of poetry was, after all, not a million miles away from the world of prose. Its constituent parts were the same old rough and ready bits and pieces that could be glimpsed strewn about the rest of the world.

Little by little other items of evidence accumulated to bring those mighty dolmens tumbling to the ground. I read somewhere that it had ...