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cordingly correlative to what must be proved.
"The substance of all the definitions" of probable cause "is a reasonable ground for belief of guilt." McCarthy v. De Armit, 90 Pa. St. 63, 69, quoted with approval in the Carroll opinion. 267 U.S. at 161. And this "means less than evidence which would justify condemnation" or conviction, as Marshall, C.J., said for the Court more than a century ago in Locke v. United States, 7 Cranch 339, 348. Since Marshall's time, at any rate, it has come to mean more than bare suspicion: Probable cause exists where "the facts and circumstance within their [the officers'] knowledge and of which they had reasonably trustworthy information [are] sufficient in themselves to warrant a man of reasonable caution in the belief that" an offense has been or is being committed. Carroll v. United States, 267 U.S. 132, 162.
These long-prevailing standards seek to safeguard citizens from rash and unreasonable interferences with privacy and from unfounded charges of crime. They also seek to give fair leeway for enforcing the law in the community's protection. Because many situations which confront officers in the course of executing their duties are more or less ambiguous, room must be allowed for some mistakes on their part. But the mistakes must be those of reasonable men, acting on facts leading sensibly to their conclusions of probability. The rule of probable cause is a practical, nontechnical conception affording the best compromise that has been found for accommodating these often opposing interests. * * * [Brinegar v. United States, 338 U.S. 160, 175-176].
