Full Idea
The ultimate warrant for moral principles cannot be found in reason. The sort of opprobrium that attaches to moral transgressions is quite unlike the sort of opprobrium that attaches to the requirements of reason.
Gist of Idea
Morality isn't based on reason; moral indignation is quite unlike disapproval of irrationality
Source
Harry G. Frankfurt (The Reasons of Love [2005], 2.5 n6)
Book Reference
Frankfurt,Harry G.: 'The Reasons of Love' [Princeton 2006], p.47
A Reaction
More like a piece of evidence than a proper argument. We may not feel indignant if someone fails a maths exam, but we might if they mess up the arithmetic of our bank account, even though they meant well.