This is Judge Hirsch's Constitutional Calendar for June 9 and it is a great one:
In 1807, Aaron Burr, former Vice President of the United States, was tried for treason. The trial was presided over by Chief Justice John Marshall. Burr participated in his own defense, but was also represented by (among others) Luther Martin, a legendary trial lawyer of that era.
On June 9, Burr made an application to the court: President Jefferson, in a proclamation to Congress the previous November 27, had referred to a letter and other papers received by him from General Wilkinson, the principal witness against Burr. Burr had good reason to believe that these papers would exculpate him in part, and would demonstrate that the case against him was a political plot to discredit and destroy him. He had applied to the Secretary of the Navy for permission for his counsel to inspect these papers, and had been denied. “Hence,” claimed Burr, “I feel it necessary ... to call upon [the court] to issue a subpoena to the President of the United States, with a clause, requiring him to produce certain papers; or in other words, to issue the subpoena duces tecum.”
Marshall labored long and hard over the order adjudicating the motion. That order concluded:
"It cannot be denied that to issue a subpoena to a person filling the exalted station of the [presidency] is a duty which would be dispensed with more cheerfully than it would be performed; but, if it be a duty, the Court can have no choice in the case. If then, as is admitted by the counsel for the United States, a subpoena may issue to the President, the accused is entitled to it of course; and, whatever difference may exist with respect to the power to compel the same obedience to the process as if it had been directed to a private citizen, there exists no difference with respect to the right to obtain it."
The principle is as old as Magna Carta: Rex non debet esse sub homine, sed sub Deus et lege. The king – or the president – is not subject to other men, but is subject to God and the law.