‘I see.’ Hitchens began to spin his chair again. ‘Diane, this isn’t evidence. It’s speculation. You need something more substantial.’

‘Well, we also came up with this –’ Fry handed him two pages of closely printed text. ‘It’s an article written by Professor Robertson and published on one of the thanatology websites.’

Hitchens ran his eye over the pages. ‘It looks deadly stuff to me.’

‘I’ve highlighted the relevant paragraphs for you, sir.’

‘So you have.’

Fry watched Hitchens read, and saw the recognition dawn on his face. She hadn’t been sure whether he’d make the connection immediately. He hadn’t listened to the tapes of the phone calls as often as she had. He didn’t know the phrases by heart, the way she did.

But, as she watched him, she knew exactly the words that her DI was reading from Freddy Robertson’s website article.

Wasn’t it Sigmund Freud who said that every human being has a death instinct? Inside every person, the evil Thanatos fights an endless battle with Eros, the life instinct. And according to Freud, evil is always dominant. In life, there has to be death. Killing is our natural impulse. The question isn’t whether we kill, but how well we do it. Without a purpose, the act of death has no significance.

For once, the flashing green light on the answering machine gave Cooper a little surge of pleasure as he walked into his flat. He pressed the button and listened to the recording before he even took his jacket off or paid attention to the cat.

‘Ah, yes,’ said the voice. ‘It’s Robertson here.’

Cooper stopped quite still in the middle of the room. Robertson? Professor Robertson? It must be, yet his voice sounded quite different. He’d lost the heartiness completely. Complacency and smugness had gone. Instead, he sounded weary and dejected. And, somehow, very small.

‘I, er … that is, I have something I need to tell you,’ said Robertson. ‘I hope you don’t mind my calling you at home, but you gave me your number. And, well, there is something …’

There was a pause on the recording. Cooper found himself listening for background noises, the way he’d listened to the tapes of the mystery caller. But there was nothing. No traffic, no voices raised in the opening verse of ‘Abide With Me’. Only the faint whisper of the professor’s breathing, slow and uncertain.

‘Strangely, it’s the one thing we never discussed,’ said Robertson. And now there was a hint of his old self again, the man Cooper had spent so much time listening to. Just a suggestion, but it was there – a sly, ironic taunting that had become all too familiar.

He moved towards the answering machine, thinking the message had ended. But not quite. There was one more thing the professor had to say.