Sherlock's eyes narrow but he bites back the scathing reply.

He watched John die. Again. And the body was never retrieved. (He knew. He went back to look. Mangled. Half eaten. No longer John, but he couldn't deny that it once had been.

"I am not accustomed to judging truths on blind hope," he says finally. He doesn't deny that what John said could be true. That he desperately wanted it to be true. But it was that desperation that caused the burden of proof to be ever that much higher. He was biased - he wanted John to be alive - and he knew it.