For the next few weeks we’ll be tackling a big subject: warrants. This week we’ll talk about general warrant requirements, next week is arrest warrants, and the following week the focus will be on search warrants. Finally, we’ll discuss what happens if police violate the warrant requirements.

The part of the US Constitution that applies here is the 4th Amendment:

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.

Okay, fine. So the amendment says that we have the right to be free from unreasonable searches and seizures. Great. And if the police want to get a warrant, they need probable cause.

So first off, what’s a search or seizure? Well, a search is government intrusion into a place or thing in which the owner has a reasonable expectation of privacy. If a cop overhears my phone conversation while I’m sitting at the adjacent table in Starbucks, that’s not a search because I can’t reasonably expect privacy. But if she grabs my phone and starts scrolling through my texts, that is probably a search.

A seizure is a meaningful interference with property or with a person’s freedom. If a cop tows my car away, that’s a seizure. If she slaps cuffs on me and hauls me off to jail? Also a seizure.

Great. But, um, what’s probable cause? The courts have been reluctant to define it too precisely, but basically it means there’s enough information that a reasonable person could deduce that evidence or contraband is likely in a particular location or that a specific person has committed a crime. It’s more than a hunch, more even than a good guess, but it’s far less than beyond a reasonable doubt, which is the standard of proof required for criminal convictions.

In simplest terms, the 4th Amendment says that in order to search or seize property or arrest someone, a police officer must have probable cause. And she must present her evidence in a sworn statement to a neutral party—a magistrate or judge—who will determine if probable cause does indeed exist. Furthermore, the warrant has to be specific about the place that’s being searched and what’s being searched for, or the identity of the person who can be arrested.

Now as it turns out, the use of warrants is more complicated than that. We’ll get to some particulars in the next two weeks.

Another thing to note now, however, is the procedure for obtaining warrants. The cop can show up in court. But almost all jurisdictions allow phone warrants as well, in which the transaction occurs via phone instead of in person. This is helpful if time is of the essence, e.g., the evidence is likely to disappear.