You're right, Jenny, I could have put this more clearly. In a court case an essence is that everyone must tell the truth. However, there is an exception in the doctrine that a person cannot make a gain through falsity or negligence - promissory estoppel or just estoppel - under which a person is not allowed to say in court what is true. An instance that comes to mind is that A asserts to B that C is his manager with power to act for him. B makes contracts with C on that basis. A later trys to renege on these contracts. A is not allowed, having induced B to make the contracts, to go to court to prove the truth that C was in fact not his manager, only a clerk. It must proceed in court as if C was A's manager, although this is untrue. Hope this is better! There are estoppels to do with land, but that is another subject! Best wishes.