From Longman Dictionary of Contemporary Englishbe a matter of principlebe a matter of principleto be something that you feel you must or must not do, because of your moral principles She couldn’t take the money. It was a matter of principle. → matter
Examples from the Corpus
be a matter of principle• Dessert is a matter of principle here.• It was a matter of principle.• First, there are matters of principle.• They might have gone higher but this is a matter of principle.• It seemed, as much as anything, to be a matter of principle. 