Obligations contracted by mere consent are exemplified by sale, hire, partnership and agency, which are called consensual contracts because no writing, nor the presence of the parties, nor any delivery is required to make the obligation actionable, but the consent of the parties is sufficient. Parties who are not present together, therefore, can form these contracts by letter, for instance, or by messenger: and they are in their nature bilateral, that is, both parties incur a reciprocal obligation to perform whatever is just and fair, whereas verbal contracts are unilateral, one party being promisee, and the other alone promisor.