My bank account is in credit.
You say that because you hope it is. (Psychological cause.)
I am unwell
You say that because you are a hypochondriac. (Psychological cause.)
“My bank account is in credit” and “I am ill” are facts about the world. You can’t be irrefutably certain of them in the way you can be irrefutably certain of a geometrical theorem: banks and accountants and doctors make mistakes. But they are questions with knowable answers. The account is either in the red or in the black; the man is either sick or he isn’t. But “You think you’re ill because you’re a hypochondriac” and “You think you have money in the bank because you hope you do” are not non- sequiturs. People do sometimes think they have illnesses when they really don’t. People do sometimes over-estimate how much money they have in the bank because they are scared of being poor. (Lewis was the opposite: he would think that he was about to go bankrupt and then find that he had several hundred pounds in his current account that he’d forgotten about.) If Andrew is the kind of person who thinks that every itch is evidence of skin cancer, then “oh, you think that because you a hypochondriac” might be a very reasonable response. But “For God sake just get the doctor to check it out” would be better.