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.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

Epistulae ad Atticum [Letters to Atticus], Book 16, Letter 7, sec. 3 (16.7.3) (44 BC) [tr. Shuckburgh (1900), # 780]
    (Source)

Defending to his brother his change in plans, based on the changing political situation in Rome.

For quotation books, this is often given in a shorter form, without the parenthetical clause about much having been written on the subject (i.e., Nemo doctus unquam mutationem consilii inconstantiam dixit esse, or the shorter English translations below.)

(Source (Latin)). Alternate translations:

No philosopher, much as has been written on this subject, has ever yet affirmed that a change of plans is the same as inconsistency.
[tr. Jeans (1880), # 117]

No wise man ever called a change of plan inconsistency.
[ed. Harbottle (1906)]

No well-informed person has declared a change of opinion to be inconstancy.
[ed. Benham (1907), 16.8]

No philosopher ever called a change of plan inconsistency, though there has been a good deal written on the point.
[tr. Winstedt (Loeb) (1913)]

Well, in all the many writings on this theme, no philosopher has ever equated a change of plan with lack of firmness.
[tr. Shackleton Bailey (1968)]