"Past consideration" is a legal concept that refers to a promise or act that occurred before the formation of a contract. In contract law, past consideration is generally not considered valid or legally enforceable. This principle is based on the idea that for a contract to be valid, there must be a mutual exchange of promises or consideration between the parties at the time the contract is formed. Past consideration does not meet this fundamental requirement for several reasons:

1. Lack of Bargain: A key element of a valid contract is that there must be a mutual exchange or bargain between the parties. In the case of past consideration, the action or promise that has already occurred lacks the element of bargaining at the time of the contract's formation. The consideration was provided without any expectation of receiving something in return.

2. Absence of Mutuality: Contracts are based on the principle of mutual assent and benefit. Past consideration typically lacks the mutuality required for a contract because it involves one party doing something without any express or implied promise of receiving something in return.

3. Preventing Unjust Enrichment: Allowing past consideration to support a contract could lead to unjust enrichment, where one party benefits from the actions or promises of the other without having to provide anything of value in return. This goes against the fundamental principles of fairness and equity in contract law.

4. Clarity and Certainty: To ensure clarity and certainty in contracts, the law requires that the terms and conditions of a contract be specific and clear. Past consideration may lack the clarity and specificity required for a valid contract.

However, there are exceptions to the rule against past consideration. For instance, if there is a subsequent agreement that acknowledges and ratifies the past consideration, it may become enforceable. Additionally, in some jurisdictions and specific situations, courts may recognize past consideration under certain circumstances, but these exceptions are limited and subject to judicial interpretation.