Introduction

Probable cause is a basic tool of Fourth Amendment law. It appears prominently in the constitutional text: “no warrants shall issue, but upon probable cause.” Police power often depends on it. In a range of settings, searches and seizures are constitutional if the police have probable cause but unconstitutional if they don't. But just how “probable” is probable cause?

The Supreme Court has declined to say. The Supreme Court has described probable cause as a “nontechnical conception” that requires proof of a “fair probability.” The Court has also said that probable cause exists when “a man of reasonable caution” would believe an offense has been committed. But the Court has refused to quantify probable cause. “The probable-cause standard is incapable of precise definition or quantification into percentages,” the Court recently declared, “because it deals with probabilities and depends on the totality of the circumstances.”