How promises are outwardly binding

If we consider only the one who has promised, he is under obligation to perform, of his own free will, that to which he wished to bind himself.…But because internal acts are not of themselves perceivable,…some degree of certainty must be established, lest there should fail to be any binding obligation. [To prevent a person from freeing himself from obligations] by inventing whatever meaning he might wish, natural reason itself demands that the one to whom the promise has been made should have the right to compel the promisor to do what the correct interpretation suggests. For otherwise, the matter would have no outcome, a condition which in morals is held to be impossible.

The measure of correct interpretation is the inference of intent from the most probable indications. These indications are of two kinds, words and implication; and these are considered either separately or together.

Words are to be understood in their ordinary sense

If there is no implication which suggests a different conclusion, words are to be understood in their natural sense, not according to the grammatical sense which comes from derivation, but according to current usage.