A concept developed by Saul Kripke in his exposition of Ludwig Wittgenstein's private language and rule-following arguments in the latter's Philosophische Untersuchungen.

In the PU Wittgenstein spent some time developing the idea that we (or at least a sceptic) can interpret our actions according to a rule that we would not want to assert that we are following. In his 1982 Wittgenstein On Rules and Priave Language: An Elementary Exposition, Kripke's sceptic attacks the assumption that we can know that by the sign "+" we have always intended the arithmetical plus function (i.e., normal addition). Kripke's skeptic claims that perhaps by the sign "+" we have always intended the quus function.

Quus is identical to plus in every respect except that if either of the two arguments on either side of the "+" sign are numerals greater than 56, then the numeral that belongs after the "=" sign is aways 5. Kripke: "The sceptic claims (or feigns to claim) that I am now misinterpreting my own previous usage. By 'plus' he says, I always meant quus" (page 9).

Wittgenstein's response to quus theorizing (the concept has become a familiar trope within the rhetoric of analytic philosophy, to be sure) begins around Philosophische Untersuchungen 199 and Philosophische Untersuchungen 202: relevant sections begin at Philosophische Untersuchungen 143 and continue all the way through the private language arguments around Philosophische Untersuchungen 294.