In the 1950s I was an athlete. Those were the days before joggers clogged the highway, so it was unusual for me to see another runner when I was training. We fell into the habit of meeting up and pounding the miles together for company.

He was stocky, barrel-chested, with a high-pitched, donnish voice and the aerodynamics of a brick. He was funny and witty and he talked endlessly, but I understood very little of what he was saying, and it became clear that he ran in order to think. He seemed to be obsessed by mathematics and biology. That much I could work out.

We had one thing in common: a fascination with Disney's Snow White and the Seven Dwarfs, especially the transformation of the Wicked Queen into the Witch. He used to go over the scene in detail, dwelling on the ambiguity of the apple, red on one side, green on the other, one of which gave death. We had both been traumatised by Walt.

On one occasion he asked me whether, in my opinion as a classical linguist, artificial intelligence was possible. After a couple of miles of silence I said that, in my opinion, it was not. And that was that.

He killed himself when an ignorant and uncouth judge gave him the choice of a prison sentence or chemical castration; and I was overwhelmed by fury at the salacious, gloating humiliation imposed on my friend, and by a sense of guilt that I did not, could not, help him; which lasted for decades, and was made only worse when the Official Secrets Act revealed his true heroism.

He died of cyanide poisoning. By his body was an apple, partly eaten. The apple was not tested for cyanide. His name was Alan Turing.

• Alan Garner's Collected Folk Tales is published by HarperCollins Children's Books.