[Cicero], Rhetoric for Herennius 4.39
The anonymous author is explaining how to employ different rhetorical techniques. He gives five examples of commutatio, of which I include just the first: it’s a sentiment which was much quoted in antiquity.
commutatio est cum duae sententiae inter se discrepantes ex traiectione ita efferuntur ut a priore posterior contraria priori profiscatur, hoc modo: ‘esse oportet ut vivas, non vivere ut edas.’
A ‘reciprocal change’ is when two sentiments which disagree with each other are expressed by a transposition, in such a way that the latter follows from the former even though it is contradictory to it, like this: ‘You should eat to live, not live to eat.’