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.
An obligation must exist between two definite parties. Both at Roman law and at common law a party to an obligation may consist of one or more persons. The number is immaterial as long as they can be grouped into two classes, one of which owes the duty and the other of which has the right to demand it. It is clear that there can not be a legal obligation which consists of a duty which one owes to himself. May not A, however, owe a duty to a group which consists of A, B and C! The common law was likely to deny the existence of an obligation in this case because its rules as to parties prevented the same person from appearing as plaintiff and as defendant. There seems to be no reason, however, why this should not be recognized as an obligation. Equity by reason of its greater freedom in dealing with parties, had no trouble in enforcing such duties. Can not an obligation exist between three or more parties by which each owes different duties to each of the other parties} Common law by reason of its rule that all of the parties must be either plaintiffs or defendants and that the defendants must be those against whom a joint judgment could be rendered in favor of all the plaintiffs, was unable to deal with a duty of this sort and was inclined to deny the existence of the obligation. Here again equity by reason of its greater liberality in its rules as to parties and by reason of * the greater flexibility of its decrees had no trouble in doing justice. No reason appears why an obligation must be limited to two parties. Whatever the number of parties to an obligation, there must be a definite number. If A has a duty which he can assert against the entire world in some specific thing or in some incorporeal right, such a duty is classed with property rather than with obligation. In some conditions of society it may arise out of status. It is also, however, excluded from the class of obligations.