Final answer:
Under certain circumstances, the initial promise can serve as valid consideration for a subsequent promise. This includes situations where there is a pre-existing duty, part payment or performance, or the doctrine of promissory estoppel applies.
Step-by-step explanation:
Under certain circumstances, the initial promise can serve as valid consideration for a subsequent promise. One such circumstance is when there is a pre-existing duty. For example, if a person is already obligated to perform a certain act, promising to perform that act does not require additional consideration. Another circumstance is when there is a part payment or part performance. If a party has partially fulfilled their obligation, their promise to complete the remaining part can be considered valid consideration. Lastly, the doctrine of promissory estoppel can also make the initial promise enforceable if the promisor should have reasonably expected the promisee to rely on it and the promisee did indeed rely on it to their detriment.