Full Idea
I will define 'quus' by x-quus-y = x + y, if x, y < 57, and otherwise it equals 5. Who is to say that this is not the function I previously meant by '+'?
Gist of Idea
'Quus' means the same as 'plus' if the ingredients are less than 57; otherwise it just produces 5
Source
Saul A. Kripke (Wittgenstein on Rules and Private Language [1982], 2)
Book Reference
Kripke,Saul: 'Wittgenstein on Rules and Private Language' [Blackwell 1993], p.9
A Reaction
Kripke's famous example, to illustrate the big new scepticism introduced by Wittgenstein's questions about the rationality of following a rule. I suspect that you have to delve into psychology to understand rule-following, rather than logic.