Isaac Asimov, “The Feeling of Power.”
That was a fun story in which humanity had forgotten how to do math without calculators and someone had begun devising the algorithms himself by studying ancient computers.
3x7 is 21.
How do you know that?
Well, I just do. I’ve tried it on a calculator any number of times and it always comes out the same.
But how can you be sure it always will?
My methods work, you see.
(At least, as best I remember the interchange.)
It always worked in the past, I expect it to work in the future - logical fallacy.