He certainly didn’t feel insane, he reflected—but then did any lunatic, ever?

Initially, he worried that he might be going crazy. But then he decided if you felt you were crazy you weren't really crazy because he had heard somewhere that crazy people didn't know they were insane.

'There are two men in me,' he thought, 'one quite sensible, the other a lunatic. But I am not concerned with that any longer... What shall I do, though, if the sensible man wins?'

“Anybody ever tell you you’re a lunatic?”
“Not in this context.”

Maybe he was crazy, he thought. It would explain everything. Insanity was good that way.

Perhaps a lunatic was simply a minority of one.

I say, Watson,’ he whispered, ‘would you be afraid to sleep in the same room as a lunatic, a man with softening of the brain, an idiot whose mind has lost its grip?’
‘Not in the least,’ I answered in astonishment.
‘Ah, that’s lucky,’ he said, and not another word would he utter that night.

His was not a small mind bothered by logic and consistency.

We didn't know if ordinary lunatics or just unstable people on the ground could feel incited to harm me.

He never went mad in your sense. My dear fellow, you're very much wrong if you think there was anything pathological about him — then. The man was brilliantly sane. His mind was as keen as a keen sword. I couldn't understand him, but I could judge of his sanity right enough.

He didn't think he was edging into dementia. He suspected he was edging into sanity, the long way around. The hard way.

He was not so anxious to prove himself right, as to be so.

The ordinary lunatic is generally a harmless, isolated case; since everyone sees that something is wrong with him, he is quickly taken care of. But the unconscious infections of groups of so-called normal people are more subtle and far more dangerous.

Perhaps a lunatic was simply a minority of one. At one time it had been a sign of madness to believe that the Earth goes round the Sun; today, to believe the past is inalterable. He might be alone in holding that belief, and if alone, then a lunatic. But the thought of being a lunatic did not greatly trouble him; the horror was that he might also be wrong.