What is the legal test for privilege from disclosure?

19 The common law principles underlying the recognition of privilege from disclosure are simply stated. They proceed from the fundamental proposition that everyone owes a general duty to give evidence relevant to the matter before the court, so that the truth may be ascertained. To this fundamental duty, the law permits certain exceptions, known as privileges, where it can be shown that they are required by a “public good transcending the normally predominant principle of utilizing all rational means for ascertaining truth”: Trammel v. United States, 445 U.S. 40 (1980), at p. 50.