The U.S. Supreme Court recognized in 1803 that in order to maintain a society governed by laws, a legal remedy should accompany each legal right. Toward this end, courts apply various remedies to ensure effective enforcement of constitutional rights. For example, courts sometimes order retrials to remedy violations of defendants' trial-by-jury or assistance-of-counsel rights. A remedy that excludes impermissibly obtained evidence from use at a criminal trial—the "exclusionary rule"—similarly protects constitutional rights. The exclusionary rule typically applies in cases involving violations by law enforcement of rights guaranteed by the Fourth or Fifth Amendments to the U.S. Constitution. It differs from remedies such as retrial, because in addition to retrospectively redressing injustice, its major aim is prospective deterrence of government misconduct. In theory, although it only actually redresses violations when probative evidence is found, the exclusionary rule also protects innocent people by deterring unwarranted privacy intrusions. The rule operates to prohibit the introduction at trial of probative evidence that would be admissible if collected in a constitutionally permissible manner. Because the excluded evidence is frequently incriminating, many believe that its application aids criminals in escaping punishment. For this reason, the rule has long been controversial. In past cases, the Supreme Court has defended the rule as a necessary corollary to the constitutional rights it protects. More recently, a division has emerged. Some justices adhere to the view of the rule as constitutionally required. Other justices express concerns about the cost to society of freeing criminals who would likely be convicted if the excluded evidence was admitted. Over the past several decades, the Supreme Court has narrowed the scope of the exclusionary rule in Fourth Amendment cases—that is, in cases involving illegal searches or seizures. The Court's 2009 decision in Herring v. United States furthers this trend. Because Herring is the first Supreme Court decision that rejects the exclusionary rule in the context of police error regarding a warrant, the decision has made news headlines and prompted debate about whether the Herring decision appropriately limits the exclusionary rule's reach. The Fourth Amendment to the U.S. Constitution provides a right "of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures." As a general rule, "reasonableness" requires law enforcement officers to demonstrate "probable cause" and obtain a warrant (unless a recognized warrant exception applies) before conducting searches or seizures. For example, under the general rule, a police officer may not arrest a person unless a judicial magistrate has issued a warrant, based on evidence establishing sufficient probable cause, for that person's arrest. Likewise, a police officer typically may not search a person's belongings without first obtaining a warrant that describes, with sufficient particularity, the property for which sufficient evidence justifies a search. The Constitution does not explicitly provide a remedy that applies when governmental actors violate a citizen's Fourth Amendment right. To deter Fourth Amendment violations, courts apply the exclusionary rule, which "is often the only remedy effective to redress a Fourth Amendment violation." In the Fourth Amendment context, the exclusionary rule requires a trial court to forbid the prosecution's use of evidence obtained as a result of an unconstitutional search or seizure. For example, if a police officer arrests a person in violation of constitutionally mandated procedures (i.e., without a warrant or a warrant exception), then the exclusionary rule requires a trial court to suppress any contraband the officer discovered during the search incident to that arrest. Although the exclusionary rule protects constitutional rights, a question remains regarding its status—that is, is it constitutionally required in the Fourth Amendment context? In past Fourth Amendment cases, the Supreme Court has stated that the exclusionary rule is "of constitutional origin." In other cases, the Court has characterized the rule as a "judicially created remedy ... rather than a personal constitutional right." This distinction affects Congress's authority to alter the exclusionary rule statutorily. Congress may not reduce a constitutionally guaranteed remedy but could potentially alter a rule that lacks constitutional status. Regardless, the Court has narrowed the exclusionary rule's reach in Fourth Amendment cases throughout the past several decades. For example, it has barred courts' use of the rule in civil cases, grand jury proceedings, and parole revocation hearings. Arguably, the most important narrowing trend has been the Court's development of the good-faith exception. The Supreme Court introduced what has come to be known as the good-faith exception in United States v. Leon . In Leon , the Court held that the exclusionary rule does not apply when police officers act with "objectively reasonable reliance" on a search warrant later found to be invalid. Language in the opinion embraced a cautionary "balancing" approach to the exclusionary rule in which the benefits of exclusion (namely any deterrence effect on unconstitutional police action) must outweigh the costs (namely the risk that a guilty person will escape justice because evidence is excluded at trial) before the Court will apply the rule to new factual circumstances. Police officers in Leon , acting on a tip about drug activity in a particular home, investigated the license plate number and connections of a man who exited the home holding a small paper sack. The officers then observed people coming and going from the residences of several people connected to that man, including the home of Leon, the respondent in the case, whom the man had listed as his employer and who had a criminal record. Based on these observations, the officers obtained a warrant from a magistrate to search three homes and several automobiles. The subsequent search uncovered illegal drugs and other evidence. At trial, a federal district court held that the warrant was not supported by probable cause; thus, the search violated the Fourth Amendment. Applying the exclusionary rule remedy, the district court suppressed the evidence of drugs found in the homes and cars. On appeal, the Supreme Court held that suppression is inappropriate in cases, such as Leon , where the violation occurred despite a police officer's "objectively reasonable reliance"—for example, on a warrant that is actually invalid. By creating an exception to the exclusionary rule, the Leon court arguably opened the door to permitting evidence in cases involving multiple types of Fourth Amendment violations. However, the Leon decision itself addressed only the particular circumstance in which a warrant exists but was invalidly issued based on insufficient probable cause. The Leon opinion, including several exceptions to the good-faith exception articulated in the case, evidences a holding that only addresses that particular context. To justify its holding, the Leon court noted the logical inconsistency between exclusion in cases involving non-police errors and the rule's traditional deterrence rationale, stating: "Penalizing the officer for the magistrate's error, rather than his own, cannot logically contribute to the deterrence of Fourth Amendment violations." Based on the Court's reliance on this rationale, one might argue that the Court did not originally anticipate an extension of the good-faith exception to cases involving police error. The Supreme Court has extended the Leon good-faith exception in relatively minor ways over the past several decades. In a 1995 case, Illinois v. Krull , the Court applied the exception where police officers had searched an auto dealer's list of licenses pursuant to a statute that courts later struck down as unconstitutional. Several years later, in Arizona v. Evans , the Court applied Leon to evidence obtained after an arrest based on a facially valid warrant that the clerk of the court had neglected to show had been quashed seventeen days earlier. Until recently, these extensions had involved police reliance on errors made by actors—for example, the clerk of the court in Evans and the legislative branch in Krull — not the police themselves. Furthermore, in a 2004 case, Groh v. Ramirez , the Court seemed to draw an explicit line between police errors and errors made by other actors. Police officers in Groh searched a home where they suspected that the owners had stored illegal weaponry. The court of appeals held that the search warrant, which a magistrate had signed but the officers had themselves prepared, violated the constitutional requirement that property to be searched be described with particularity; thus, the officers' search violated the homeowners' Fourth Amendment rights. On appeal, the Supreme Court declined to apply the good-faith exception to the exclusionary rule, because it found that the officers' search pursuant to a warrant that failed to list property to be searched was not a "reasonable" mistake. In so holding, the Court stressed that the officer in Groh was himself responsible for the Fourth Amendment violation. However, only two years after Groh , the Court declined to find any distinction between police error and third-party errors. In Michigan v. Hudson , it held that police officers' violation of the "knock and announce" rule did not trigger the exclusionary rule. Knock and announce, an "ancient" procedure derived from common-law, constitutional, and statutory sources, protects occupants' privacy by requiring police officers to wait a short while after knocking and announcing their presence before entering a residence for which they have a warrant. The rule is viewed as a less stringent requirement than the warrant or probable cause requirements under the Fourth Amendment, and the Hudson court noted that it is "unnecessary" in various circumstances. Because the Court limited its opinion in Hudson to knock and announce violations, it was unclear after that case whether the Court would extend the good-faith exception to more serious police errors, such as those involving warrants or warrant exceptions. In Herring v. United States , a 2009 case, the Supreme Court for the first time applied the good-faith exception in a case involving police error regarding a warrant. Officers arrested the defendant, Bennie Dean Herring, outside of an impound lot where Herring had come to retrieve an item from his truck. An officer at the lot, recognizing Herring, called the county warrant clerk to determine whether an outstanding arrest warrant applied to him. The warrant clerk found no such warrant but agreed to inquire about warrants in a neighboring county.