In all such oaths we are not to attend to the mere form of words, but the true design and intention of them.

[Semper autem in fide quid senseris, non quid dixeris, cogitandum.]

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

De Officiis [On Duties; On Moral Duty; The Offices], Book 1, ch. 13 (1.13) / sec. 40 (44 BC) [tr. Cockman (1699)]
    (Source)

(Source (Latin)). Alternate translations:

In obligations of faith, it is the meaning always, not the words that are to be considered.
[tr. McCartney (1798)]

In a promise, what you thought, and not what you said, is always to be considered.
[tr. Edmonds (1865)]

In a promise, what you mean, not what you say, is always to be taken into account.
[tr. Peabody (1883)]

A promise must be kept not merely in the letter, but in the spirit.
[ed. Harbottle (1906)]

In the matter of a promise one must always consider the meaning and not the mere words.
[tr. Miller (1913)]

You should always, in a matter of trust, think of what you mean, not of what you say.
[tr. Edinger (1974)]