Executive privilege is a shield against attempts by Congress and courts to pierce the executive branch’s defenses to gain access to testimony or documents. It’s rooted in the notion of separation of powers. As the Supreme Court noted when it held that President Richard Nixon could not refuse to turn over the Watergate tapes, executive privilege preserves “the supremacy of each branch within its own assigned area of constitutional duties.” The doctrine rests on a recognition that the president and subordinate officials require some confidentiality in order to operate effectively. Due to the paucity of judicial decisions and absence of relevant constitutional text, the scope of such confidentiality is hotly disputed between Congress and any administration, especially under divided government. (As an official in President Barack Obama’s administration, I experienced the great joy of squaring off with GOP congressional staff over numerous requests for documents and testimony.)

But once Comey left office and signaled a willingness to testify, this ceased to be about separation of powers. Congress isn’t trying to pry loose information. Rather, an ex-employee with a story to tell wants to tell it to Congress.

The fact that Comey wants to talk is just as important as the fact that he’s no longer in government. Executive privilege can be invoked to protect a reluctant former official from having to testify. This makes sense, because otherwise Congress could wait until an official stepped down and then force her to talk about matters that she couldn’t be commanded to discuss while serving. This is not hypothetical – in 2007, President George W. Bush invoked executive privilege in response to a House Judiciary Committee subpoena issued to former White House Counsel Harriet Miers.