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 it is generally said that the duty must be one which is reduceable to a money value. This was the theory of the Roman law; but it seems to have arisen at a time when the formula which the praetor submitted to the judex required him to condemn the defendant in a certain sum of money if he found certain facts to be true and otherwise to acquit him. The fact that at common law the obligee was given a money judgment in case of a breach of duty on the part of the obligor made this view seem to be the correct one. It is, however, simply carrying over a detail of procedure into the general theory of obligations. Since equity can give affirmative relief by specific performance and negative relief by injunction, it follows either that the reduction to a money value is not an essential element of obligation or that obligation has a different meaning in equity from its meaning at law. It might also be suggested that this theory of obligation leaves out such courts as the ecclesiastical courts of medieval England or the courts of honor on the continent which attempted to enforce the performance of duties without regard to their money value. To amount to an obligation the duty must involve some right which the law can recognize; but it should not be limited to the legal rights which are reduceable to a money value. When obligation is limited to a money value by the words of the rule, the limitation is promptly evaded by legal fictions which ascribe money value to legal rights • to which, in fact, no money value can be ascribed.