As is shown elsewhere 1 each promise in a bilateral contract must be sufficient consideration for the other, or both promises are invalid. Accordingly if either promise is too indefinite for enforcement, or if either promise for any reason is insufficient consideration, both promises fail. But a promise that was originally too indefinite, may by performance become definite and as the other party to the bargain must be regarded as continuously assenting to receive such performance in return for his own promise, a valid unilateral contract arises on receipt of such performance.2 So a bilateral agreement illegal in the inception, because it was made on Sunday, may by performance on one side on Monday become a valid unilateral contract.3 So if the promise on one side of a bilateral agreement is invalid as consideration because it promises nothing detrimental to the promisor or beneficial to the promisee, though both promises are consequently invalid, yet if performance is made of the counter-promise and that performance was something detrimental to the promisor or beneficial to the promisee, the promise which was itself insufficient as consideration, thereupon becomes binding, since sufficient consideration has now been received for it, and it is no longer necessary that the promise which was insufficient as consideration, should serve as such. This is because only promises need consideration. Transfers, or other actual performances may be made without consideration. Unilateral contracts must be supported by consideration only on one side. So in the case supposed, the performance which has been rendered needs no consideration though the promise to give it originally did. Since the performance has been rendered under no mistake of fact, it cannot be recovered back, and being received as the consideration for a promise, that promise now becomes binding. It certainly cannot he in the mouth of the promisor to say that since the promise, which he has made is of such Blight value he will not perform it at all though he has been paid for doing so.4 So while a promise void for incapacity of the promisor will not support a counter-promise, if the void promise is actually performed, the performance may become sufficient consideration to support the counter-promise.5 And other instances may be found where a bilateral agreement originally unenforceable gives rise, when performed on one side, to a binding unilateral contract.6
1Supra, Sec. 103. 2See supra, Sec. 49.
3 See infra, Sec.Sec. 1704, 1707.