This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
In order to amount to an obligation the legal duty must be to perform or to forebear a definite act or a number of definite acts. If one party is legally bound to do everything that the adversary party may demand, the relation is one of status; and at both Roman law and common law it is grouped with the law of persons and not with the law of obligations. Duties of this sort in a more or less modified form, grow out of family relations or out of slavery where that institution is recognized.