 The fundamental principle is that a witness shall give in evidence the whole truth that he knows concerning the matter in dispute and that the parties to the dispute are entitled to have that evidence given.The rules which regulate and which, in certain exceptional cases, restrict the giving of evidence are the growth of practice and of the rulings of judges, occurring mainly within the last two to three centuries (see the judgment of Parke B in the case of "The Queen v. Ryle", 9 M. & W., 244).The rule of Public Interest Immunity which excludes evidence, the requiring of which would be contrary to public policy, as may occur in relation to the conduct of the business of a state department, is an instance.