Question

What was the painful thought that disturbed Leacock

Answer 1

He used to have a painful thought that if Todd owes him a dollar and has forgotten it.

It is possible, indeed it is theoretically probable, that there must be men to whom the narrator owes a dollar which he has forgotten because he realised.

That if he starts remembering and listing all whom he owes a dollar or so, the list might get longer and longer