Probable Cause – Definition

Cite this article as:"Probable Cause – Definition," in The Business Professor, updated December 2, 2019, last accessed October 26, 2020,

Back To: Legal Disputes: Civil and Criminal Law

Probable Cause Definition

Probable cause is a criminal law requirement that must be met in order for police to carry out an arrest, conduct a search, obtain a warrant, or seize property. The probable cause requirement arises from the U.S. Constitution’s Fourth Amendment, which provides for citizens’ right to freedom from unnecessary government intrusion into their homes, persons, and businesses.

Probable cause is pertinent in two criminal law parts. First, it’s compulsory the police have probable cause before searching an individual or property, and also before they can make an arrest. Next, the court must see a probable cause in order to believe the defendant committed the crime before prosecution is carried out.

A Little More on What is Probable Cause

When there is a search warrant, police must only search for the things described in the warrant, despite the fact that they can seize evidence of other crimes or contraband found. But supposing the search is illegal, any evidence found is subjected to the “Exclusionary rule” and can’t be utilized against the defendant in court.

Landmark Probable Cause Case

In the development of probable cause and search warrants, Illinois v. Gates is a landmark case. In May 1978, Bloomingdale, Illinois police department got a letter from an unknown source highlighting explicit information about the defendants’ plans – Gates and others – to move drugs from Florida to Illinois. The police got a search warrant from a judge based on the anonymous letter and a signed affidavit. When Gates got home, the Bloomingdale police ransacked the car, recovered about 350 lbs of marijuana and also more marijuana, as well as weapons in the residence of Gates.

But, the Illinois Circuit Court referred to the search as unlawful as the affidavit didn’t provide enough evidence for establishing enough reason, leading to the exclusion of the evidence gotten based on the warrant. The case was moved to the Supreme Court which reversed the ruling of the Illinois court. Ruling in favor of the State of Illinois, the Supreme Court declined the Aguilar-Spinelli test. This test is a judicial guideline which the Supreme Court established for examining a search warrant’s validity or an arrest with no search warrant on the basis of information provided by an anonymous tip or confidential informant. The Aguilar-Spinelli test’s two prongs state that, when a magistrate signs a search warrant requested by the police, he or she must be kept abreast of:

  •         the reasons for supporting the conclusion that the informant is trustworthy and credible; and
  •         certain underlying circumstances which the informant relies upon.

On the court, the Supreme Court used a “totality-of-the-circumstances” standard, as more evidence of Gates’ involvement in drug trafficking existed asides the letter by itself. For example, Florida was prominent for illicit drugs, and Gates’ stay in a motel for just one night aroused suspicion. Furthermore, the Court agreed that the anonymous letter by itself wouldn’t be probable cause for getting a warrant, while the Aguilar-Spinelli’s “reliability” prong was unlikely to be satisfied, at any point, by an anonymous tip.

Generally, the Supreme Court’s verdict, in this situation, lowered the threshold of probable cause by ruling that it can be established by “fair probability” or a “substantial chance” of criminal activity, as against a better-than-even chance.

References for “Probable Cause” › FAQs › General Questions › Insights › Laws & Regulations

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