From Longman Dictionary of Contemporary Englishbe another thing/matterbe another thing/matterused to suggest that something may not be true, possible, easy etc, after mentioning something that is It is true that his programme is original, though whether it is funny is quite another matter. It is one thing to talk about ‘involving the students’; it is quite another thing to actually do this. → another
Examples from the Corpus
be another thing/matter• Defending a U. S. Senate seat is another matter.• Whether I understood them was another matter.• Whether they will be allowed to evict their unwelcome, unsavoury, tenants, from belfries and elsewhere, is another matter.• But for many of us, reading is another thing altogether.• But the administration that has now begun work in Washington will be another matter altogether.• Real art is another matter and, despite recent genuflections towards Rembrandt, a rarity becoming rarer.• And that is another matter entirely.• Indeed it can: but whether the argument would carry any weight is another matter entirely. 