"No, wait ... I'll tell you something," said Zaphod. "I freewheel a lot. I get an idea to do something, and, hey, why not, I do it. I reckon I'll become President of the Galaxy, and it just happens, it's easy. I decide to steal this ship. I decide to look for Magrathea, and it all just happens. Yeah, I work out how it can best be done, right, but it always works out. It's like having a Galacticredit card which keeps on working though you never send off the cheques. And then whenever I stop and think — why did I want to do something? — how did I work out how to do it? — I get a very strong desire just to stop thinking about it. Like I have now. It's a big effort to talk about it."
Zaphod paused for a while. For a while there was silence. Then he frowned and said, "Last night I was worrying about this again. About the fact that part of my mind just didn't seem to work properly. Then it occurred to me that the way it seemed was that someone else was using my mind to have good ideas with, without telling me about it. I put the two ideas together and decided that maybe that somebody had locked off part of my mind for that purpose, which was why I couldn't use it. I wondered if there was a way I could check.
"I went to the ship's medical bay and plugged myself into the encephelographic screen. I went through every major screening test on both my heads — all the tests I had to go through under government medical officers before my nomination for Presidency could be properly ratified. They showed up nothing. Nothing unexpected at least. They showed that I was clever, imaginative, irresponsible, untrustworthy, extrovert, nothing you couldn't have guessed. And no other anomalies. So I started inventing further tests, completely at random. Nothing. Then I tried superimposing the results from one head on top of the results from the other head. Still nothing. Finally I got silly, because I'd given it all up as nothing more than an attack of paranoia. Last thing I did before I packed it in was take the superimposed picture and look at it through a green filter. You remember I was always superstitious about the color green when I was a kid? I always wanted to be a pilot on one of the trading scouts?"
Ford nodded.
"And there it was," said Zaphod, "clear as day. A whole section in the middle of both brains that related only to each other and not to anything else around them. Some bastard had cauterized all the synapses and electronically traumatised those two lumps of cerebellum."
Ford stared at him, aghast. Trillian had turned white.
"Somebody did that to you?" whispered Ford.
"Yeah."
"But have you any idea who? Or why?"
"Why? I can only guess. But I do know who the bastard was."
"You know? How do you know?"
"Because they left their initials burnt into the cauterized synapses. They left them there for me to see."
Ford stared at him in horror and felt his skin begin to crawl.
"Initials? Burnt into your brain?"
"Yeah."
"Well, what were they, for God's sake?"
Zaphod looked at him in silence again for a moment. Then he looked away.
"Z.B.," he said.
At that moment a steel shutter slammed down behind them and gas started to pour into the chamber.
"I'll tell you about it later," choked Zaphod as all three passed out.