Why are certain terms defined in the Constitution?

Perhaps these terms need no definition because these terms were commonly known to the Framers. If one reviews the notes of the 1787 Convention, one finds that no one questioned terms like Natural Born Citizen. NBC was defined and known through Vattel's work, Law of Nations, a work referenced in Article 1 Section 9 Pp 10. In a review of The Documentary History of the Ratification, a series of 40+ texts published by U Chicago Press, I found no questions about the terms. Other terms that seem strange or undefined to modern ignorants were also known to the Framers. Militia was known to be all persons capable of keeping and bearing Arms, primarily based on fitness and in some cases an age group, but still, leaving the membership to a general class, those capable. Questions concerning terms in the Constitution come primarily from those wishing to corrupt our system. They attempt to manipulate the words, calling the Constitution a living document but a living document that can be changed on whim does not need a method of amendment.