Full Idea
If some societies with divergent moral systems merely confront each other, having no use for the assertion that their own systems are true and the others false except to mark the system to which they adhere, then relativism is a true theory of morality.
Gist of Idea
If moral systems can't judge other moral systems, then moral relativism is true
Source
report of Bernard Williams (The Truth in Relativism [1974]) by Philippa Foot - Moral Relativism p.3
Book Reference
Foot,Philippa: 'Moral Dilemmas' [OUP 2002], p.33
A Reaction
'Having no use for' an assertion is not the same as the assertion being impossible. Some liberal cultures refuse to criticise others because their highest value is tolerance, even when the target culture wholly contradicts the critics' other values.