Full Idea
Putting forward a valid argument isn't necessarily going to succeed in getting someone to see things your way, because if they don't accept the conclusion, they ask which premises they should reject, or whether an illegitimate assumption is being made.
Gist of Idea
Valid arguments can be rejected by challenging the premises or presuppositions
Source
Michael Martin (Interview with Baggini and Stangroom [2001], p.136)
Book Reference
Baggini,J/Stangroom,J: 'New British Philosophy' [Routledge 2002], p.136
A Reaction
Valid arguments are still vital. It is just that good philosophers realise the problem noted here, and spend huge stretches of discussion on establishing acceptance of premises, and showing that there are no dodgy presuppositions.