Response or Comment
The dictum that if there be nothing in a rule flatly contradictory to reason the law will presume it to be well founded, and that the office of the judge is "jus dicere and not jus dare", is responsible for much agony of construction and tortious logic on the part of courts torn by desire to evade it in the interest of modern ideas of right. There is a trilogy of accepted legal principles which it has been particularly difficult for the courts to adhere to in spirit or to repudiate in letter. They are the propositions, that for a promise to be enforcible a consideration must emanate from the promisee, that doing what one is already legally bound to do is not a consideration, and that one is legally bound to perform a contract according to its terms. In other words, doing what one has already contracted to do is not consideration for a promise made on condition or in contemplation of such performance. Under this rule, if A. has contracted to do something for B., his actual performance of that promise can not be consideration for a new promise by B. or a collateral promise by C. There is no lack of real application of the rule.
Waite, John B. "Performance of an Existing Obligation as Consideration for a Promise." Mich. L. Rev. 16, no. 3 (1918): 182-4.