The Fourth Amendment to the US Constitution seems straightforward on its face: At its core, it tells us that our “persons, houses, papers, and effects” are to be protected against “unreasonable searches and seizures.” Before any government agent can perform a search or seizure, they must first obtain a warrant, based on “probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.”

For courts, however, arriving at satisfactory interpretations of these principles has been anything but straightforward. During a recent conversation on Twitter with Orin Kerr, Jacob Appelbaum, and Jennifer Granick, we discussed the fact that interpretations that involve physical spaces and objects can generally be understood by the average citizen, as our intuitions make good guides when deciding what is and is not private in the physical, tangible world. This mutual understanding between citizen and government helps us preserve the protections articulated within the Fourth Amendment through our ability to spot government overreach and abuse.

Our intuitions about privacy run into difficulties, however, when our use of technology forces us to use metaphors to describe new situations and possibilities. We grew comfortable with, for example, talking about the Internet as a sort of “place” we would go, which was easier, perhaps, than trying to describe packets of data being routed between servers. Before too long, courts were making arguments about computer “trespass,” as if we were actually setting foot on someone’s computer. These inexact metaphors can have serious consequences in the real (physical) world, which is especially true for our current thinking about the Fourth Amendment.

In the 1967 case of Katz v. United States, the Supreme Court called this mutual understanding a “reasonable expectation of privacy,” and made it the standard for deciding when Fourth Amendment protections apply — a standard we continue to follow today. This standard depends on our understanding of what we expect to be private and what we do not. In Katz, for instance, the defendant made a telephone call not from his home, but from a public phone booth, which could be seen by anyone on the street, including the police. The fact that Katz closed the door to the phone booth indicated to the Court that he expected his conversation to be private, just as if he were using the telephone in his own home. This logic depends on an accepted understanding of walls and doors as physical and symbolic means of keeping eavesdroppers away from our private conversations. But what happens when technology takes us out of the realm of physical walls and doors, causing us to lose at least some ability to understand the boundaries the Fourth Amendment sets on government searches and seizures?

For example, it is well-established — and generally understood — that the contents of any sealed letters or packages we send through the Postal Service are considered private, and they “can only be opened and examined under [a] warrant, issued upon [] oath or affirmation, particularly describing the thing to be seized, as is required when papers are subjected to search in one’s own household.” The only exceptions to this rule are the observations of the letter’s properties one can observe without opening it, such as its size, its weight, and the address information written on it. Can the same be said about our email? One can’t touch or otherwise physically manipulate an email message like one written on paper, but we still tend to think of email messages as a contemporary analogue to “letters.” Does it therefore follow that we have the same expectation of privacy in our email messages as we do our letters and packages?

The Sixth Circuit Court of Appeals thought so. In United States v. Warshak, the court observed that “[g]iven the fundamental similarities between email and traditional forms of communication, it would defy common sense to afford emails lesser Fourth Amendment protection,” and held that “a subscriber enjoys a reasonable expectation of privacy in the contents of emails that are stored with, or sent or received through, a commercial ISP.” (Internal citations omitted).

Everyone — including judges — is drawn to the use of metaphors and analogies when it comes to applying Fourth Amendment doctrine to the less-than-tangible. But all metaphors, however clever, are imperfect, and can be used to hide important details that may be more difficult to understand.

Returning to the email example, while most of us may not fully understand the processes behind email transmission, we have a pretty good idea how letters and packages get delivered, mainly due to the fact that the key components of the operation are tangible and subject to physical inspection. We thus gain some measure of confidence from this understanding that violations of our expectation of privacy in our letters would be highly infeasible for the government to pull off at any sort of scale. But we will likely not have that level of confidence with respect to our email messages, due in large part to our inability to inspect the process in a tangible or meaningful way. Does this affect our expectations of privacy regarding our email messages?

The generalized version of this question becomes especially important when we consider the effect of the “third-party doctrine,” which, as expressed in Smith v. Maryland, holds that “a person has no legitimate expectation of privacy in information he voluntarily turns over to third parties.” Thus, a person’s phone billing records, the items at issue in Smith, were merely collections of “numerical information” “voluntarily conveyed” by the defendant to the telephone company, and he could therefore not have a reasonable expectation of privacy in those records. This early articulation of the third-party doctrine has since expanded into a number of different areas, including our use of rapidly advancing technologies, like smartphones, the Internet of things, and automated cars. Some courts have held, for example, that the highly detailed location information our smartphones constantly emit, and which is collected by cell phone companies as cell-site location data, falls under the third-party doctrine, and we therefore have no reasonable expectation of privacy in that data.

This is where we start to lose the thread of the Fourth Amendment’s intent. It is probable that the Constitution’s drafters would agree that our willing — and knowing — disclosure of information to third parties may affect its status under the Fourth Amendment, but it is another thing entirely to say that our partial (or mis-) understanding of a technology alone erodes our expectations of privacy in it. While I am sure most of us understand, at least implicitly, that our smartphones share some information with our phone companies, it is not at all clear that this hazy understanding immediately translates into a general waiver of privacy expectations in our smartphones. Or our smart refrigerators. Or our smart cars.

Some part of this issue can be attributed to the fact that the “reasonable expectation of privacy” test and the third-party doctrine are showing their age, and courts are having a harder time trying to fit mid-20th century doctrine around a 21st century world. But it is also clear that our hazy understanding of the details behind our rapidly advancing technologies causes us to rely too heavily on imperfect metaphors. This may be fine for general conversation, but when it comes to our civil liberties, our comprehension of the details matters.