Probable cause is a legal term used in most common law American criminal law jurisdictions that denotes the standard by which a police officer may conduct a personal or property search, or an arrest. This term comes from the Fourth Amendment of the United States Constitution.

The most widely held definition is "a reasonable belief that a crime has been committed". However, this definition is controversial. A proposed alternate definition is "reason to believe that an injury had criminal cause".

In the United States a probable cause hearing is the preliminary hearing that typically takes place after arraignment and before a serious crime goes to trial; the judge is presented with the basis of the prosecution's case and the defendant is afforded full right of cross-examination and the right to be represented by legal counsel. If the prosecution cannot make out a case of probable cause the court must dismiss the case against the accused. See also: evidentiary hearing.

Summary of the Controversy

The controversy over the classic definition of probable cause can be catagorized in the following areas:

  • Syntactic problems: definition vs. usage
    • The definition: "reason to believe that a crime has been committed" is inconsistant with the most common usage (Google link - observe number of matching pages), i.e., "The officer had probable cause to believe a crime had been committed." (simple synonym for "reason").
  • Law/Crime circularity
    • The common definition makes reference to either "crime" or "violations of law", but the United States Constitution is defining the law. If law is "that which is crime", and crime is "violations of law", then this forms a logical circle. There must exist an a priori notion of "that which is crime" before codification of law, i.e., "a crime is when a person causes injury to victim". Without a tight coupling between the notion of crime and "victims with injury", a large number of "victimless crimes" potentially might arise (and, civil libertarians argue, in fact have)
  • Improper sequence of events
    • According the the Fourth Amendment, the arrest sequence should be
      1. Civilian complaint ("oath or affirmation")
      2. Probable Cause determined
      3. Specific warrant issued (an important check against the execution of standing orders)
      4. Arrest made
      However, most arrests today are warrantless, and probable cause hearings are almost always made after the fact.
  • Weak Burden of proof
    • To prove "reason to believe that a crime has been committed", all that is needed is the statement of opinion of one person, the arresting officer. With "reason to believe that an injury has criminal cause", a more rigorous burden of proof is required, that is, demonstratable facts connecting the accused with the victim's injury.
  • Undermines Bill of Rights mission of protecting rights of all citizens, including those accused of crimes.

This all must be understood in the context of the times of the American Revolution. These conditions are detailed in the Declaration of Independance. Civil libertarians argue that the corruption of this definition over time, deviating from what the original authors had intended, is one primary factors which has allowed near police state conditions to arise in America.

Please see the Lawful Arrest FAQ for a more rigorous discusson of these topics.

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