Full Idea
'Obeying a rule' is a practice. And to think one is obeying a rule is not to obey a rule. Hence it is not possible to obey a rule 'privately': otherwise thinking one was obeying a rule would be the same thing as obeying it.
Gist of Idea
One cannot obey a rule 'privately', because that is a practice, not the same as thinking one is obeying
Source
Ludwig Wittgenstein (Philosophical Investigations [1952], §202)
Book Reference
Wittgenstein,Ludwig: 'Philosophical Investigations', ed/tr. Anscombe,E. [Blackwell 1972], p.81
A Reaction
The core of the Private Language argument. But if I drive on the right erroneously thinking it is the law, I can still make progress until I meet someone.