Full Idea
The maxim of honesty: never put forward a philosophical theory that you yourself cannot believe in your least philosophical and most commonsensical moments.
Gist of Idea
Honesty requires philosophical theories we can commit to with our ordinary commonsense
Source
David Lewis (On the Plurality of Worlds [1986], 2.8)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.135
A Reaction
I take it as important that this test is according to the philosopher's commonsense, and not according to some populist idea. This would allow, for example, for commonsense to be sensitive to scientific knowledge, or awareness of the logic.