For the last two weeks on the blog, we’ve examined the legal framework that exists in U.S. law for interactions between police officers and members of the public. These laws have to strike a delicate balance: on the one hand, they must ensure that all citizens as well as their property remain safe and secure from criminals and criminal activity; on the other hand, they need to ensure that people suspected of criminal activity are treated fairly and accorded their full rights under the law. The system may not always run perfectly–after all, with human nature, personal fallibilities and split-second decisions as part of the mix, there’s bound to be some difficulties–but, as I hope we’ve been showing, the laws do an excellent job of maintaining this fine line according to reasonable, common-sense dictates.

For a good example, let’s take a closer look at the concept of “probable cause,” which can be a contentious area in some cases for the way it might seem to rely on intuitive, borderline-subjective readings of a situation, even though it is very firmly rooted in the law. It goes back, actually, to the Fourth Amendment of the Constitution, which protects against “unreasonable searches and seizures.” As we stated earlier, probable cause is what’s considered necessary under the law for a police officer to obtain a warrant–either for search and seizure or for a person’s arrest. Police also need probable cause to arrest without a warrant, and prosecutors need it to charge someone with a crime.

As we can see already, there are several complex levels to probable cause as an important concept in the pursuit of justice, and its definition is no exception: probable cause is said to exist when circumstances and facts known to a police officer would lead an average person to believe that a crime was, will be, or has been committed. This is also the case in searches–where facts in the knowledge of a police officer would suggest to a reasonable person that a given location is either the scene of a crime or that it contains evidence pertaining to a crime.

If it seems like a good deal of this is left up to an officer’s discretion, remember that their decision must be based on facts or evidence, and never on intuition or a “hunch.” That may constitute “reasonable suspicion,” as we mentioned last week, but never probable cause.

In situations where a warrant isn’t necessary–if a crime is committed in front of a police officer, for instance, or when someone gives verbal consent to a search–police officers must still operate under assumptions of what is “reasonable,” i.e., they must never rely on anything but facts and evidence. Indeed, when public safety is threatened, they must rely steadily on their recourse under the law. It is a powerful tool, and when used correctly, it is overwhelmingly a force for good.