The term, "probable cause", comes right out of the words of the 4th Amendment to our US Constitution, which states as follows:
"The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."
The police do not need “probable cause” to stop you. All they need is “reasonable suspicion”. What is the difference? Frankly, the only difference is one of degree. An officer’s suspicion might be reasonable, if the facts and circumstances are such that he/she believes that a crime was committed (or that you might possess a weapon that poses a threat to them). Under those circumstances, not only can an officer stop you, but he/she can frisk you, as well. In this case, Terry v Ohio, the US Supreme Court held that a pat-down can be done, pursuant to an officer’s “reasonable suspicion”, and that a weapon or other contraband can be seized without violating the 4th Amendment.
OK, so the 4th Amendment is what sets forth the standard of probable cause, and the requirements of warrants, and the right to be secure from “unreasonable searches and seizures”. So what then, is “probable cause”?
It is not satisfying to say that it is more than mere “reasonable suspicion”, but that is as good as any. In Brinegar v US, our US Supreme Court defined probable causes as “where the facts and circumstances within the police officers’ knowledge, and of which they have reasonably trustworthy information, are sufficient in themselves to warrant a belief by a person of reasonable caution, that a crime is being committed.”
This somewhat squishy standard became even squishier, 34 years later, when the Supreme Court held in Illinois v Gates lowered the threshold of probable cause, by ruling that a “substantial chance” or even a “fair probability” of criminal activity could establish probable cause; it need not even be a better than even chance.
Over the years, particularly since the 1970’s and up to the present day, the trend has been for the Courts to expand the definition of “reasonable suspicion” and to liberalize (i.e., for the benefit of police officers) the definition of “probable cause”, to allow ever greater intrusions into what had previously been areas of personal privacy.