(This is the second of a short series of posts on my new forthcoming article, Applying the Fourth Amendment to the Internet: A General Approach, forthcoming in the Stanford Law Review. For the first post, click here.)

  So imagine you're trying to apply the Fourth Amendment to the Internet so that it plays the same role in Internet crime investigations that the Fourth Amendment plays in traditional physical investigations. How can you do that?

  The first step is to realize that the Fourth Amendment in the physical world is based on a fundamental distinction between inside surveillance and outside surveillance. If something occurs outside, the police are free to watch it: The Fourth Amendment doesn't regulate them. On the other hand, if something occurs inside, the Fourth Amendment presumptively applies: Barring special circumstances such as consent, the Fourth Amendment regulates government access to that inside space.

  The line between inside and outside can get a little tricky, to be sure. In fact, many of the most interesting cases consider exactly where the line falls (think Kyllo or the open fields/curtilage cases). But that distinction is essential to the Fourth Amendment in the physical world. It creates a zone that the police can monitor, mostly involving transactional information about where people are and what they are doing, while carving out a private zone where the police can't go without special circumstances, mostly involving the substance our of private thoughts, writings, and expression.

  Now turn to the Internet. Online, the distinction between "inside" and "outside" no longer makes a lot of sense. Some new distinction is needed to replace the function of the outside/inside distinction.

  My article proposes that the online distinction between content and non-content information should replace the physical distinction between inside and outside surveillance. The contents of communications, such as e-mail messages, subject lines, and remotely stored files, should be presumptively protected by the Fourth Amendment. Absent special circumstances such as consent or posting documents on the Internet, the Fourth Amendment should regulate government access to contents. On the other hand, non-content information, such as IP addresses mail header information other than the subject line, should not be protected.

  I think this approach nicely translates the Fourth Amendment to the Internet because content and non-content information are network substitutes for inside and outside. To see this, you need to think about how networks work and what functions they serve. Networks are means for transporting information or property from one place to another. Instead of transporting information or property yourself, you sit back and the network does that work for you remotely. The non-content information is the information in the network that is generated and used to deliver the package: It records where the package went, when, how big it was, and the like. In contrast, the content information is the package itself: It is the actual information that the person sent.

  Replacing the inside/outside distinction with the content/non-content distinction leads to a technology-neutral Fourth Amendment because the latter is a network substitute for the former. The non-content information — that information generated to deliver the package — is the network substitute for what would have been open to public observation and therefore "outside" in a physical environment with no network. The content information is the network substitute for what would have been hidden from public observation and therefore "inside" in a physical environment.

  Replacing inside/outside with content/non-content maintains the same basic function of the legal distinction in light of the shift from physical space to network space.