Full Idea
Philosophical arguments are never incontrovertible - well, hardly ever. Their purpose is to help expound a position, not to coerce agreement.
Clarification
'Incontrovertible' means 'impossible to challenge'.
Gist of Idea
Arguments are nearly always open to challenge, but they help to explain a position rather than force people to believe
Source
David Lewis (Lewis: reduction of mind (on himself) [1994], p.419)
Book Reference
'A Companion to the Philosophy of Mind', ed/tr. Guttenplan,Samuel [Blackwell 1995], p.419
A Reaction
A bit over-cautious, perhaps. Most philosophers are converted to a position when they hear a single key argument, though it is probably 'tipping the balance' of previous discussions.