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.