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THE amount of philosophical writing available on any of the principles studied in this essay is small compared to the vast literature that exists on such subjects as the principle of the uniformity of nature or the principle of causality. Even less has been said about the nature, status and origin of the type of principle that forms the subject of our inquiry. This ought not to seem surprising once we realize that our principles are supplementary to those universal methodological principles-often referred to also as a priori postulates or metaphysical doctrines presupposed by science-which have attracted so much philosophical interest and discussion. Our principles which are principles within science are secondary in relation to the more fundamental principles which precede science and which guarantee the possibility of discourse in general.