‘It’s the transition from black putrefaction to butyric fermentation that causes the main source of odour. If you ever smell it, you won’t want to eat blue cheese again for a while. The body is drying out by then, and the exposed surfaces turn a little mouldy from fermentation. Just like your cheese will, if you leave it in the fridge too long.’

Cooper remembered he had some Blue Stilton in the fridge at home. He’d better throw it away, because he didn’t think he was going to eat it now. He’d only be thinking of a combination of Billy McGowan’s tattooed arms and butyric fermentation.

His attention beginning to stray, Cooper picked up a book that had caught his eye.

‘Do you collect antiquarian books, sir?’

‘Well, it’s hardly antiquarian. The book has no intrinsic value. It simply relates to my field of interest.’

‘I see.’

Cooper opened the book to the title page. What field of interest exactly? He knew the name of the author – everyone did, if only by reputation. But he’d never met anyone before who admitted to reading his books, let alone having one in the house. He flicked through the pages cautiously. He felt as though he might be corrupted by something if he read the words. He’d half-expected to find illustrations – dark, shocking pen-and-ink drawings between the chapters. But there weren’t any.

‘Yes, the Marquis de Sade,’ said Robertson, watching him with that smile again. ‘But not one of his, er… more celebrated titles, I’m afraid. It isn’t terribly easy to find.’

‘I’ve never heard of it,’ admitted Cooper, turning back to the cover. The book was called La Marquise de Gange.

‘One of his very last works,’ said Robertson. ‘It was published a year before de Sade died. The Marquis was seventy-three years old by then, and had been locked up in an insane asylum for the past ten years.’

Cooper put the book back on the shelf.

‘Don’t you want to know how La Marquise de Gange relates to my interests?’ asked Robertson.

‘Do I need to ask? I’m sure you’re going to tell me.’

Robertson laughed and put a hand on his shoulder.

‘Dissection,’ he said.

Cooper felt the pressure of the professor’s fingers on the layer of muscle that covered the bones of his arm. The fingers moved slightly, as if parting the sinews and blood vessels to touch the deepest part of him. He had a momentary realization that the professor really could see inside him, and knew what his body looked like from the inside out.