"But it's not a joke, is it? The voice never says or does anything to make you better. To help you get beyond whatever it is you need to get beyond."

He taps his own head with his finger tips. "Like I said, it's protecting itself. So, why would a random voice want to protect itself at the expense of the person carrying the voice? I mean usually, voices work to protect you, or pick at sore spots. This one... does more somehow."

Sherlock had been joking about a brain transplant. Maybe he wasn't wrong.