Full Idea
Unless it is possible to provide some criterion by which one may decide between conflicting intuitions, a mere appeal to intuition is worthless as a test of a proposition's validity.
Gist of Idea
Moral intuition is worthless if there is no criterion to decide between intuitions
Source
A.J. Ayer (Language,Truth and Logic [1936], Ch.6)
Book Reference
Ayer,A.J.: 'Language, Truth and Logic' [Penguin 1974], p.141
A Reaction
It is a bit much to expect a 'proof' of its 'validity'! If moral judgements are reflected in consequences, then reliable intuitions (i.e. wisdom) could be demonstrated by getting it right (for happiness, or flourishing).