(Sektion 202: Philosophische Untersuchungen von Ludwig Wittgenstein).
Perhaps one of the most-discussed aphorisms in the entire Philosophische Untersuchungen, in this section Wittgenstein finally asserts a positive thesis regarding the problem of rule-following (this is indeed a rare moment in his ouevre. Following a rule, Wittgenstein claims, is a eine Praxis,...
and -- we cannot follow a rule merely be believing that we are doing so, to follow a rule, one must involve more than their own mental states and isolated intentions -- there must be some sort of social assent that a rule is being followed too (though this social aspect of rule-following needn't necessarily involve other persons: one can follow a rule alone in their bedroom, it only must hold that there is some sort of social custom or practice whereby we can assert that this rule is indeed being followed (otherwise we are all unknowingly following all kinds of rules all the time -- and that can't be right)). Of course, this leeds right into the next sentence where Wittgenstein immediately asserts that there can be no such thing as logically-private rule-following (see private language argument).
(See Philosophische Untersuchungen 199 for more information on this problem in the context of Wittgenstein's thought.)
202. Darum ist 'der Regel folgen' eine Praxis. Und der Regel zu folgen glauben ist nicht: der Regel folgen. Und darum kann man nicht der Regel 'privatim' folgen, weil sonst der Regel zu folgen glauben dasselbe wäre, wie der Regel folgen.
202. Therefore 'following a rule' is a practice. And believing one is following a rule is not following a rule. And for that reason one cannot follow a rule 'privately', because otherwise believing one was following a rule would be the same as following a rule.