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In criminal law and criminal procedure, no term is used more often than “probable cause.”

To give just one example, my Massachusetts Criminal Law Sourcebook, which contains all of the state’s statutory and procedural rules, mentions “probable cause” over 340 times.

Despite its ubiquitous use, actual definitions for “probable cause” are few and far between.

Here are a few quotes taken from Supreme Judicial Court (SJC) cases that deal with the meaning of probable cause:

SJC Case 1

As the standard is most often formulated, probable cause exists where, at the moment of arrest, the facts and circumstances within the knowledge of the police are enough to warrant a prudent person in believing that the individual arrested has committed or was committing an offense.

Commonwealth v. Storey, 378 Mass. 312, 321 (1979)

SJC Case 2

If the arresting officers in this case had sufficient information to constitute probable cause to believe, and did believe, that any one of the defendants had committed a felony, even though not in their presence, they had the right to arrest that defendant without a warrant…The police were not required to possess information sufficient to establish beyond a reasonable doubt that a defendant arrested by them had committed a felony. It was enough if they had information which reasonably permitted a conclusion that the defendant had probably committed a felony.

Commonwealth v. Snow, 363 Mass. 778, 788 (1973)

SJC Case 3

While a warrant may issue only upon a finding of “probable cause” this of course means less evidence than would justify a finding of guilt and the finding may rest upon evidence, such as hearsay, not legally competent in a criminal trial, but there must be some support for the affiant’s belief in the credibility of the informant and the reliability of the information. 

Commonwealth v. Lillis, 349 Mass. 422, 424

When defining “probable cause” the SJC often cites to the US Supreme Court. Here’s what the USSC has said about probable cause throughout the years,

USSC Case 1

Whether that arrest was constitutionally valid depends, in turn, upon whether, at the moment the arrest was made, the officers had probable cause to make it — whether, at that moment, the facts and circumstances within their knowledge and of which they had reasonably trustworthy information were sufficient to warrant a prudent man in believing that the petitioner had committed or was committing an offense.

Beck v. Ohio, 379 U.S. 89 (1964)

The Supreme Court, in Beck, goes on to say,

The rule of probable cause is a practical, nontechnical conception affording the best compromise that has been found for accommodating . . . often opposing interests. Requiring more would unduly hamper law enforcement. To allow less would be to leave law-abiding citizens at the mercy of the officers’ whim or caprice.

Id. See also Brinegar v. United States, supra, at 338 U. S. 176.

USSC Case 2

[T]he central teaching of our decisions bearing on the probable cause standard is that it is a practical, nontechnical conception. In dealing with probable cause, as the very name implies, we deal with probabilities. These are not technical; they are the factual and practical considerations of everyday life on which reasonable and prudent men, not legal technicians, act.

Illinois v. Gates, 462 U.S. 213, 231 (1983)


[As a] fluid concept–turning on the assessment of probabilities in particular factual contexts–[probable cause is] not readily, or even usefully, reduced to a neat set of legal rules.