“Until I believe you,” Fowler said over him. “If it was possible to glance at a man’s writing and say This is a child murderer, we’d have squads of you people looking over samples from everyone in the land.”

“Then thank God it isn’t,” Joel said. “And I didn’t say he was a child murderer. I said there was something horrible about him and he liked to hurt people, and I guessed children because—” Because he’d felt something vile and squirming. “Because that sort of personality targets the weak, the helpless: that’s part of the fun. His hand stank of it and if you can’t see that, it’s not my fault.”

“I can’t see it, because it’s just handwriting. And in my view, the best way to judge a man’s character from his writing is if you already know who he is.”

Joel clenched his fist, and felt the twitch in his left arm that suggested he’d tried to clench both. “What’s your idea? That I’ve memorised samples of every lunatic murderer’s handwriting in case a police officer under a false name asks me to look at a random piece of paper?”

“Of course not.”

“Then what are you suggesting?”

“I don’t know!” Fowler said, with sudden, explosive frustration. “I want to know how you come up with this stuff, how you work it, because you have presented me with an absurd claim. You didn’t simply say that Molesworth had this or that characteristic: you recoiled as though I’d presented you with a dead rat. Therefore, you knew.”

“I knew what he was, not who he was!”

“That isn’t possible.”

“Well, it is sodding possible, because I do it. I just look at handwriting,” Joel said savagely. “I think, What kind of person would I be if I wrote like that? I imagine being the person who wrote like that, and then I tell you what I feel like. That’s the big secret you’re after: I put myself behind their pen, rather than in their shoes. I don’t know why it works—Christ, I don’t even know that it works at all, except that everyone tells me I’m right. And I certainly can’t teach you how to do it, any more than you could teach a colourblind man to see in colour.”

Fowler stared at him. Joel glared back. “Don’t look at me like that. It’s not my fault.”

“If you’re unqualified—”

“Who’s supposed to qualify me? Was I wrong about any of your letters?”

“But you’re asking for money to do something akin to palm reading. Drawing conclusions and making things up.”