No well-informed person has declared a change of opinion to be inconstancy.

[Nemo doctus unquam mutationem consilii inconstantiam dixit esse.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher
Epistulae ad Atticum [Letters to Atticus], Book 16, Letter 7 (16.7) (59-54 BC)

Alt. trans.: No philosopher ever yet -- and there has been a great deal written upon the subject -- defined a mere change of plan as vacillation. [Nemo doctus umquam (multa autem de hoc genere scripta sunt) mutationem consili inconstantiam dixit esse.]

Often mis-cited as Letter 8.