Executive privilege is the power of the president of the United States and other members of the executive branch of the United States Government to resist certain subpoenas and other interventions by the legislative and judicial branches of government in pursuit of information or personnel relating to confidential communications that would impair governmental functions. The power of Congress or the federal courts to obtain such information is not mentioned explicitly in the United States Constitution, nor is there any explicit mention in the Constitution of an executive privilege to resist such requests from Congress or courts. The Supreme Court of the United States has ruled this privilege may qualify as an element of the separation of powers doctrine, derived from the supremacy of the executive branch in its own area of Constitutional activity. The Supreme Court confirmed the legitimacy of this doctrine in United States v. Nixon in the context of a subpoena emanating from the judiciary, instead of emanating from Congress. The Court held that there is a qualified privilege, which once invoked, creates a presumption of privilege, and the party seeking the documents must then make a 'sufficient showing' that the 'presidential material' is 'essential to the justice of the case'. Chief Justice Warren Burger further stated that executive privilege would most effectively apply when the oversight of the executive would impair that branch's national security concerns. Regarding requests from Congress (instead of from the courts) for executive branch information, as of a 2014 study by the Congressional Research Service, only two federal court cases had addressed the merits of executive privilege in such a context, and neither of those cases reached the Supreme Court. In addition to which branch of government is requesting the information, another characteristic of executive privilege is whether it involves a 'presidential communications privilege' or instead a 'deliberative process privilege' or some other type of privilege. The deliberative process privilege is often considered to be rooted in common law, whereas the presidential communications privilege is often considered to be rooted in separation of powers, thus making the deliberative process privilege less difficult to overcome. Generally speaking, presidents, congresses and courts have historically tended to sidestep open confrontations through compromise and mutual deference in view of previous practice and precedents regarding the exercise of executive privilege. Deliberative process privilege is a specific instance of the more general principle of executive privilege. It is usually considered to be based upon common law rather than separation of powers, and its history traces back to the English crown privilege (now known as public-interest immunity). In contrast, the presidential communications privilege is another specific instance of executive privilege, usually considered as being based upon separation of powers, and for that reason it is more difficult to overcome than deliberative process privilege. A significant requirement of the presidential communications privilege is that it can only protect communications sent or received by the president or his immediate advisors, whereas the deliberative process privilege may extend further down the chain of command. In the context of privilege assertions by United States presidents, law professor Michael Dorf has written: 'In 1796, President George Washington refused to comply with a request by the House of Representatives for documents related to the negotiation of the then-recently adopted Jay Treaty with the Kingdom of Great Britain. The Senate alone plays a role in the ratification of treaties, Washington reasoned, and therefore the House had no legitimate claim to the material. Therefore, Washington provided the documents to the Senate but not the House.' President Thomas Jefferson continued the precedent for this in the trial of Aaron Burr for treason in 1809. Burr asked the court to issue a subpoena duces tecum to compel Jefferson to testify or provide his private letters concerning Burr. Chief Justice John Marshall, a strong proponent of the powers of the federal government but also a political opponent of Jefferson, ruled that the Sixth Amendment to the Constitution, which allows for these sorts of court orders for criminal defendants, did not provide any exception for the president. As for Jefferson's claim that disclosure of the document would imperil public safety, Marshall held that the court, not the president, would be the judge of that. Jefferson refused to personally testify but provided selected letters. In 1833, President Andrew Jackson cited executive privilege when Senator Henry Clay demanded he produce documents concerning statements the president made to his cabinet about the removal of federal deposits from the Second Bank of the United States during the Bank War. During the period of 1947–49, several major security cases became known to presidents. There followed a series of investigations, culminating in the famous Hiss-Chambers case of 1948. At that point, the Truman Administration issued a sweeping secrecy order blocking congressional efforts from FBI and other executive data on security problems. Security files were moved to the White House and Administration officials were banned from testifying before Congress on security related matters. Investigation of the State Department and other cases was stymied and the matter left unresolved.