What legal principle prevents the withdrawal of a promise when the promisee has relied on it?

Promissory estoppel is a significant legal principle that ensures a promise cannot be withdrawn if the promisee has relied upon it in a way that was foreseeable and reasonable. This principle is particularly relevant in situations where, although a formal contract may not exist, one party has made a clear promise, and the other party has taken action based on that promise.

For instance, if someone makes a promise to provide funds for a project and the other party begins to invest time, resources, or money into that project based on the promise, the promisor may be legally barred from withdrawing their promise if it would cause harm to the promisee. This protects the promisee's reliance and promotes fairness by holding the promisor accountable for their word when it leads to another party making significant commitments.

Understanding this principle is critical as it helps uphold the integrity of commitments made in various contexts, even when a formal contractual relationship is not established.