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Definitions of Probable Cause

The precise meaning of "probable cause" is somewhat uncertain. Most academic debates over the years have centered around the differences between "more probable than not" and "substantial possibility". The former involves the elements of certainty and technical knowledge. The latter involves the elements of fairness and common sense. There's more adherents of the latter approach, but how do you define common sense. Supreme Court case law has indicated that rumor, mere suspicion, and even "strong reason to suspect" are not equivalent to probable cause. Over the years, at least three definitions have emerged as the best statements:

Probable cause is where known facts and circumstances, of a reasonably trustworthy nature, are sufficient to justify a man of reasonable caution or prudence in the belief that a crime has been or is being committed. (reasonable man definition; common textbook definition; comes from Draper v. U.S. 1959).

Probable cause is what would lead a person of reasonable caution to believe that something connected with a crime is on the premises of a person or on persons themselves. (sometimes called the nexus definition; nexus is the connection between PC, the person's participation, and elements of criminal activity; determining nexus is the job of a judicial official, and it's almost always required in cases of search warrants, not arrest warrants).

Probable cause is the sum total of layers of information and synthesis of what police have heard, know, or observe as trained officers. (comes from Smith v. U.S. 1949 establishing the experienced police officer standard).

There are of course, other definitions, and it's a fact of life in criminal justice that different judicial officials use different definitions. Judges will always have the last word on probable cause. Police will use whatever judicial official is available, preferred, or the court system may have a rotating duty roster for judges or magistrates to sit for nothing but warrants. The Good Faith Exception to the Exclusionary Rule protects the police to some degree in errors made by magistrates (where an appeals court rules that the magistrate signed off too leniently on probable cause). The Sixth Amendment also requires a person arrested without a warrant be brought before a magistrate without delay. This is called First Appearance, and it involves a judicial affirmation of probable cause. Many jurisdictions also still have something called the Preliminary Hearing, and it involves the determination of whether there are reasonable grounds to believe someone is guilty (not reasonable doubt, but jacked-up probable cause). Modernized court systems combine the First Appearance and Preliminary Hearing into one Probable Cause Hearing (eliminating the redundancy).

There are a few things from other areas of the law that have relevance for the law of probable cause. One of these is the area of privacy. A "search" is in many ways a violation of privacy, a quest for something. Therefore, the Katz definition of privacy (expectation of privacy) prevails and in many ways supplements the particularity requirement (searches cannot be exploratory in hopes of finding something; they must be calculated, looking for something specific). Under Katz, only things a person clearly expects or deems private are protected; anything on display or in a public place is not protected. The area of electronic surveillance is also relevant. States cannot generally give their officers more power than the federal government allows when it comes to technology, but there are loosened restrictions on consent and different definitions of private (e.g., email) under wiretapping law. A "seizure" involves a dispossession of a person's exercise of dominion or control of a thing; the detaining of their body in the case of arrest. The only things that should be seized, as a general rule, are items within plain piew or under the immediate control of a suspect (the Chimel Rule), but in some cases (under conspiracy, racketeering, and asset forfeiture laws), a person can be dispossessed of things faraway and distant (like Swiss Bank Accounts) if the items are instrumentalities or proceeds of the crime. The law of seizure generally recognizes two precedent-setting cases:

Brinegar v. U.S. 338 US 160 (1949) -- (Probable Cause to Arrest) -- provides details on how to determine if a crime has been or is being committed.

Carroll v. U.S. 267 US 132 (1925) -- (Probable Cause to Search) -- provides details on the belief that seizable property exists in a particular place or on a particular person.

Sources of Probable Cause