Our Constitutional system of separation of power and checks and balances provides that the members of each branch of government be protected from legal consequences for performing their constitutionally mandated functions. Thus, Article I of the Constitution explicitly immunizes from arrest all members of Congress “during their attendance at the Sessions of their respected Houses, and in going to and returning from the same.” This immunity, though limited, protects legislators from arrest for actions for which ordinary citizens could be prosecuted. This limited immunity does not put them “above” the law, since it is the law itself that provides the immunity.

Judges, too, are immunized from not only from criminal prosecution, but also from civil liability for actions taken within their judicial authority. This is how the Supreme Court put it in Stump v. Sparkman (in which a young woman sued the circuit judge who had tricked her into being involuntarily sterilized by misinforming her that it was an appendectomy!): “The governing principle of law is well established, and is not questioned by the parties. As early as 1872, the court recognized that it was ‘a general principle of the highest importance to the proper administration of justice that a judicial officer, in exercising the authority vested in him, [should] be free to act upon his own convictions, without apprehension of personal consequences to himself.’”

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Prosecutors, too, have limited immunity for actions undertaken within their prosecutorial authority. It should come as no surprise, therefore, that the president — as head of the executive branch — cannot be prosecuted for acts that he or she is authorized to take pursuant to Article II of the Constitution. Article II of the Constitution explicitly allocated to the resident the authority to “take care that the Laws be faithfully executed.”

This grant of power authorizes the president to decide who should be investigated and prosecuted and who not. presidents including John Adams, Thomas Jefferson, Abraham Lincoln, Franklin Roosevelt, John Kennedy John Neely KennedyMORE and Barack Obama Barack Hussein ObamaMichelle Obama and Jennifer Lopez exchange Ginsburg memories Pence defends Trump's 'obligation' to nominate new Supreme Court justice The militia menace MORE have exercised this authority by instructing the Justice Department to investigate and prosecute individuals or groups — and not to investigate or prosecute others. Some have exercised that authority wisely, others not. But none have committed the crime of obstruction of justice by trying to influence prosecutorial decisions.

President George H.W. Bush stopped an investigation in its tracks — an investigation that could have pointed directly to criminal action by him during the Iran Contra scandal — when he pardoned Casper Weinberger and five others crucial witnesses that could have pointed the finger at him. The special prosecutor, Lawrence Walsh, was furious at this decision, claiming that it was intended to, and did, stop the investigation. Yet no one suggested that President Bush be charged with obstruction of justice, because in pardoning those witnesses he was exercising his constitutional authority under Article II.

President Andrew Johnson was impeached for firing Secretary of War Edwin Stanton in violation of a statute passed by Congress. He was acquitted by the Senate, and the Supreme Court subsequently declared the statute unconstitutional as impinging on the president’s power to fire members of the executive branch.

Even former director of the FBI James Comey said that the president had the authority to fire him, for any reason or no reason. Now President Trump’s political opponents are seeking to have the special counsel psychoanalyze the president to determine whether his motives were pure, mixed or corrupt. Nearly all presidential actions are motivated by mixed intentions, ranging from self-aggrandizement to political benefit to partisan advantage and to patriotism. If a president or a senator or a judge acts within his or her constitutional authority their motives should not be probed by prosecutors even if they suspect unsavory ones.

If a president’s actions, on the other hand, are unlawful — as President Nixon’s clearly were when he told subordinates to lie to the FBI and pay hush money — good intentions (they are hard to imagine in Nixon’s case) would not be a defense. For purposes of the criminal law, presidents must be judged by the lawfulness or unlawfulness of their acts, not by the motivations that underlay them.

My argument, unlike that of President Trump’s lawyer, is not that a president can never be charged with obstruction of justice. It is that he cannot be charged with that crime if his only actions were constitutionally authorized. This distinction is central to our system of separation of powers and checks and balances.

Alan M. Dershowitz is the Felix Frankfurter Professor of Law, Emeritus, at Harvard Law School and author of “Trumped Up: How Criminalizing Politics is Dangerous to Democracy.” Follow him on Twitter @AlanDersh and on Facebook @AlanMDershowitz.