Past consideration, or a promise made in return for something that has already been performed, is generally not supported by law because it lacks the element of a bargained-for exchange. For a contract to be valid, there must be mutual assent, an offer, acceptance, and consideration. Past consideration doesn't involve a contemporaneous exchange of promises or performances, making it challenging to establish a clear contractual obligation. Courts often require consideration to be present at the time of contract formation to ensure fairness and prevent potential exploitation.