Full Idea
The mistake is to think that whatever 'grounds' for a moral judgement may have been given, someone may be unready, indeed unable, to make the moral judgement, because he has not got the attitude or feeling.
Gist of Idea
The mistake is to think good grounds aren't enough for moral judgement, which also needs feelings
Source
Philippa Foot (Does Moral Subjectivism Rest on a Mistake? [1995], p.192)
Book Reference
Foot,Philippa: 'Moral Dilemmas' [OUP 2002], p.192
A Reaction
This is roughly the Frege-Geach problem for expressivism, of how we still make moral judgements about situations where we ourselves are entirely disinterested (such as ancient historical events).