In contracts, the right passeth, not only where the words are of the time present, or past, but also where they are of the future: because all contract is mutual translation, or change of right; and therefore he that promiseth only, because he hath already received the benefit for which he promiseth, is to be understood as if he intended the right should pass: for unless he had been content to have his words so understood, the other would not have performed his part first. And for that cause, in buying and selling, and other acts of contract, a promise is equivalent to a covenant; and therefore obligatory.