The level of justification the Fourth Amendment requires before police may arrest someone or search a place - more than a hunch, and less than proof.
Probable cause is the standard that separates a suspicion the law will act on from one it will not. It is the threshold for an arrest and for most searches, and it sits well below the standard for a conviction: proof beyond a reasonable doubt is what a prosecutor must eventually establish, while probable cause is only what is needed to begin.
The Supreme Court has repeatedly declined to reduce it to a percentage or a formula, describing it instead as a practical, common-sense judgment about whether the facts and circumstances known at the time would lead a reasonable person to believe that an offence has been committed, or that evidence of one will be found in a particular place. Two features of that description do most of the work in practice. It is judged on what was known at the time, so evidence discovered afterwards does not supply it retroactively. And it is judged on the whole picture rather than on each fact in isolation, which is why an argument that dismantles each individual fact separately often fails.
Below probable cause sits a lower standard, reasonable suspicion, which permits a brief investigative stop and a limited pat-down for weapons but not an arrest or a full search. Confusing the two is the most common source of misunderstanding about what police were permitted to do in a given encounter: a lawful stop does not automatically make what followed lawful, and each step of an encounter is assessed on its own justification.
When probable cause was absent, the usual remedy is not the dismissal of the case but a motion to suppress - an application to exclude the evidence that the unlawful search or arrest produced. Whether excluding it ends the prosecution depends entirely on whether anything else supports the charge, which is why "the search was illegal" and "the case is over" are different propositions.
Whether probable cause existed is a question for a lawyer and a judge, not one to argue at the roadside or in an interview room; it is litigated afterwards, on a written motion, against a record. What is worth preserving in the meantime is the detail: what was said, in what order, what officers were told, who else was present, and whether any recording exists. If a case rests substantially on physical evidence or on a statement, whether that evidence survives a suppression motion is frequently the whole case, and it is the first thing a defence lawyer assesses.
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