It is crucial to understand that the foreign intelligence system as it currently exists fails to require both adequate targeting and adequate oversight. The system allows intelligence agencies to gather an enormous amount of information "incidental" to any investigations. And it does so with minimal court and Congressional oversight. If the revelations of the past two days have taught us anything, it is that revision of our foreign intelligence surveillance system is a constitutional necessity. If the Fourth Amendment is to have any meaning, Congress must untangle the current web of broad authorizations and broad secrecy that allows the government to escape judicial accountability for its acts.

First, there is the question of whom the surveillance targets. PRISM spies on Americans. The Director of National Intelligence emphasized yesterday that PRISM targets only " non-U.S. persons located outside the United States ." But the press release also acknowledges that "information about U.S. persons" may be "incidentally acquired" in such pursuits. Targeting is not the same as collecting; the program may "target" foreign persons, but "acquire" information on Americans.

The current scope of this "incidental" surveillance will shock most Americans. Before 2008, the law limited "incidental" surveillance by limiting primary surveillance. The government had to show probable cause that its surveillance target was the agent of a foreign power, and that the facility being watched was about to be used by that target. You could be incidentally observed if you communicated with a targeted foreign agent, but otherwise foreign communications were likely to be unmonitored.

But in 2008, the FISA Amendments Act (FISAAA) changed this. The government now does not need to show probable cause that the target is a foreign agent. It need only have a "reasonable belief" that the target is located outside of the United States. The new version of FISA does not require the government to identify its targets; it does not require the government to identify the monitored facilities; and the purpose of foreign intelligence gathering attaches to the whole surveillance program, not the individual investigation. That is to say: the FISA Amendments Act permits the government to obtain a single court order through which it can monitor thousands, or even millions, of people. The scope of "incidental" surveillance thus vastly expanded as Congress lowered the requirements for spying on the primary target.

Such a system will inevitably sweep in untold numbers of Americans who communicate with foreigners. And because the government need have only a "reasonable belief" that the target is outside the United States--which it is interpreting according to the Washington Post as a 51% chance that the target is outside the U.S.--this system will undoubtedly sweep in purely domestic communications as well.