You practically need a scorecard to keep up with all the conflicts between Congress and President Donald Trump over executive privilege. At last count, a federal district court has required Trump’s accountants to pass along documents connected to his taxes. Meanwhile, the administration is withholding the returns themselves on the basis of a claim of privilege. And Trump has directed former White House counsel Donald McGahn not to respond to a House subpoena requiring him to testify about episodes he described to special counsel Robert Mueller’s investigation.


Fortunately, there’s a handy-dandy principle you can rely upon to determine when Congress should win and when the president should.


It’s this: Congress should get the information it needs to do its job of legislation and oversight, and the president should be able to withhold what is necessary for him to do his job well.


If you prefer prediction to principled explanation, it’s a safe bet that the lower courts will follow this principle. (The U.S. Supreme Court is something else again.)


In practice, that means Congress can get the records from Trump’s accountant - because it has legislative oversight over presidential finances and even potential impeachment. The same goes for Trump’s tax returns, which Congress is entitled to by federal law.


But McGahn shouldn’t and can’t be compelled to appear before Congress, because the president needs to be able to keep confidential his conversations with his senior advisers in order to run the country effectively.


For what it’s worth, I didn’t invent this principle. The Supreme Court did, in the landmark 1974 decision of United States v. Nixon. Writing for a unanimous court, Chief Justice Warren Burger held:


“A President and those who assist him must be free to explore alternatives in the process of shaping policies and making decisions, and to do so in a way many would be unwilling to express except privately. These are the considerations justifying a presumptive privilege for Presidential communications. The privilege is fundamental to the operation of Government, and inextricably rooted in the separation of powers under the Constitution.”


Burger’s basic idea here is twofold. First, as a pragmatic matter, the president can’t do his job unless he gets the freely expressed, unfettered advice from his senior aides. If those advisers know that they could be subpoenaed and required to testify under oath about what they’ve told the president, either by a court or by Congress, they will be constrained in their willingness to express themselves fully and freely. Some degree of privilege exemption from testimony is required to make the presidency function well.


The second element of Burger’s logic is constitutional. The pragmatic requirement to let the president function effectively can be grounded in the Constitution. It’s not in any specific clause, to be sure. Rather, the separation of powers between the different branches of government can be construed to protect each branch’s ability to do its own job.


The president’s need to get good advice from his advisers certainly extends to the White House counsel. Almost no presidential adviser is more central. Technically, the White House counsel isn’t the president’s personal attorney, but the attorney for the presidency itself. So attorney-client confidentiality arguably doesn’t apply to the job. Regardless, however, the White House counsel functions as a lawyer advising a client, albeit an official, abstract one. That relationship demands candor, respect - and confidentiality.


The strongest argument for forcing McGahn to testify is that Trump already waived executive privilege by allowing McGahn to speak to Mueller. But the Trump administration has a non-crazy response to that argument: It is different to let one executive employee speak to another member of the executive branch (which Mueller is, as part of the Justice Department) than it is to let the former White House counsel be subpoenaed by a separate branch of government.


The argument sounds cheap, because McGahn presumably already told Mueller everything he knows.


But at the level of constitutional principle, it has some value. Trump isn’t wrong to say that if McGahn can be subpoenaed, that could create a precedent for future White House counsels. On balance, the principle of executive privilege should likely be read to stop Congress from subpoenaing McGahn.


Yet it’s important to notice that Burger’s formulation doesn’t unduly elevate the president’s interests above those of Congress or the courts. In the Nixon case, the court actually balanced executive privilege against the interests of the criminal justice system, and held that the president had to hand over the Watergate tapes under subpoena. In other words, the judiciary’s interests outweighed those of the executive in that situation.


Similarly, Congress has the well-established authority to subpoena materials that fall within its legitimate exercise of legislative authority under the Constitution.


That’s why Congress can and should get Trump’s tax-related records, from his accountants and from the IRS. Congress has the power to make laws regulating the president’s finances. It also has oversight authority over the president, up to and including the constitutional power to impeach.


Crucially, nothing in Trump’s tax records from before he was president has any connection to advice the president might receive from his senior staff - nor any destructive impact on the functioning of the presidency itself. The logic of executive privilege simply doesn’t apply to documents produced by Trump the private citizen. Burger’s rationale for executive privilege has no application.


The principle of executive privilege isn’t perfect by any means. The Supreme Court could also change it. But it’s worked all right for 45 years - and it’s worth applying it now.


Noah Feldman is a Bloomberg Opinion columnist. He is a professor of law at Harvard University and was a clerk to U.S. Supreme Court Justice David Souter. His books include “The Three Lives of James Madison: Genius, Partisan, President.”