Final answer:
The 'good faith' exception allows unlawfully obtained evidence to be used in court if the police had an objectively reasonable belief that they were acting within the law, such as executing a search they thought was valid, despite later finding the search to be unconstitutional.
Step-by-step explanation:
According to the good faith exception, unlawfully obtained evidence may be used when an officer's actions are founded upon their objectively reasonable reliance on something which later proves to be invalid or unconstitutional. The key concept here is that if law enforcement officers act with the belief that they are following legal procedures, such as executing a search warrant they assume to be valid, evidence they discover can be used in court, even if the warrant is later found to be defective. This exception assumes the officers' good faith reliance on the legal system's checks, such as the issuance of a warrant by a judge.
The good faith exception exists alongside other exceptions like the 'inevitable discovery' doctrine, where it's established that the evidence would have been found legally regardless of the error. Also, it's important to note the broader impact of the exclusionary rule, established in cases like Mapp v. Ohio and Weeks v. United States, which generally prohibits the use of evidence obtained from an unlawful search or seizure in a criminal trial.