fundamental to the idea of a limited government accountable to the people. The separation of powers principle is particularly noteworthy in regard to the Congress. The Constitution declares that the Congress may exercise only those legislative powers "herein granted" within Article I (as later limited by the Tenth Amendment). It also, by implied extension, prohibits Congress from delegating its legislative authority to either of the other branches of government, a rule known as the nondelegation doctrine. However, the Supreme Court has ruled that Congress does have the latitude to delegate regulatory powers to executive agencies as long as it provides an "intelligible principle" which governs the agency's exercise of the delegated regulatory authority. That the power assigned to each branch must remain with that branch, and may be expressed only by that branch, is central to the theory. The nondelegation doctrine is primarily used now as a way of interpreting a congressional delegation of authority narrowly, in that the courts presume Congress intended only to delegate that which it certainly could have, unless it clearly demonstrates it intended to "test the waters" of what the courts would allow it to do. Although not specifically mentioned in the Constitution, Congress has also long asserted the power to investigate and the power to compel cooperation with an investigation. The Supreme Court has affirmed these powers as an implication of Congress's power to legislate. Since the power to investigate is an aspect of Congress's power to legislate, it is as broad as Congress's powers to legislate. However, it is also limited to inquiries that are "in aid of the legislative function"; Congress may not "expose for the sake of exposure". It is uncontroversial that a proper subject of Congress's investigation power is the operations of the federal government, but Congress's ability to compel the submission of documents or testimony from the president or his subordinates is often-discussed and sometimes controversial (see executive privilege), although not often litigated. As a practical matter, the limitation of Congress's ability to investigate only for a proper purpose ("in aid of" its legislative powers) functions as a limit on Congress's ability to investigate the private affairs of individual citizens; matters that simply demand action by another branch of government, without implicating an issue of public policy necessitating legislation by Congress, must be left to those branches due to the doctrine of separation of powers. The courts are highly