In Roman law and the civil legal systems descending from it, the condictio indebiti is a legal action whereby a plaintiff may recover what he has paid the defendant by mistake; such mistaken payment is known as solutio indebiti. This action does not lieif the sum was due ex aequitate, or by a natural obligation; if he who made the payment knew that nothing was due, for qui consulto dat quod non debet, praesumitur donare.