It's easy to see how one metaphor (a phone number is like an envelope) leads to another (an email's metadata is like a phone number), but over time this sort of reasoning breaks down. Is the collection and analysis of all the metadata of American's calls really like the writing on the outside of an envelope? Orders of magnitude different is an understatement. Metadata this deep can "reveal sensitive political information, showing, for instance, if opposition leaders are meeting, who is involved, where they gather, and for how long," Jane Mayer writes in The New Yorker. "Such data can reveal, too, who is romantically involved with whom, by tracking the locations of cell phones at night."

Simply put, the quantity of this data makes it nothing like the information scrawled on an envelope. The Court could certainly apply its "reasonable person" standard and find that today's NSA programs go so far beyond the sort of surveillance in Smith that the Fourth Amendment prohibit them, that our constant conveyance of information constitutes a major shift in what we can reasonably expect to remain private. But it could, just as easily, find that the NSA phone-record collection is not materially different than a pen register -- the technology at issue in Smith -- and green-light the whole thing.

But what of the PRISM program revealed last night? That does include access to email contents. The details on this are extremely muddled at the moment, making any serious legal analysis quite strained. At least for the moment, it seems that many of the legal questions will hinge on whether the Internet companies voluntarily participated in the program. If that is the case, that might be enough to satisfy any Fourth Amendment concerns.

If they didn't know, a lot may depend on the program's use of "selectors" to "produce at least 51 percent confidence in a target's 'foreignness' " -- people, that is to say, who are not protected by the Fourth Amendment. FISA requires that targeted individuals are "reasonably believed to be located outside the United States." Is a standard of 51 percent confidence sufficiently reasonable? What percent would be sufficient? The Court could answer that question, but, if it follows its own reasoning in Clapper, you're first going to have to find an American who knows that he or she was targeted, and good luck with that.

Just because technology makes something possible doesn't mean government has to do it. Laws can constrain practice; in a healthy democratic system, society should be able to make judgment calls about what that practice looks like -- which technologies are on the table and which are off. But the example of the Fourth Amendment demonstrates why this is so difficult: Legislators and judges have to make these decisions half-blind, without full knowledge about what possibilities they are foreclosing, and which they are inadvertently leaving open. It's not clear what the best way to proceed would be, nor if any could be better; maybe the Court's muddling approach to the Fourth Amendment will, in the end, provide it with the flexibility it needs to address surveillance-state creep.