elementary outline of the whole treatment of the subject. For a comprehensive view is often requiredj the details hut seldom. " To the former, then — the main heads — we must continually return, and must memorize them so far as to get a valid conception of the facts, as well as the means of discovering all the details exactly when once the general outlines are rightly understood and remembered ; since it is the privilege of the mature student to make a ready use of his conceptions by referring every one of them to elementary facts and simple terms. For it is impossible to gather up the results of continuous diligent study of the entirety of things, unless we can embrace in short formulas and hold in mind all that might have been accurately expressed even to the minutest detail. <; Hence, since such a course is of service to all who take up natural science, I, who devote to the subject my continuous energy and reap the calm enjovment of a life like this, have prepared for you just such an epitome and manual of the doctrines as a whole. " In the tir^t place. Herodotus, you must understand what it is that words denote, in order that by reference to this we may be in a position to test opinions, inquiries, or problems, so that our proofs may not run on untested ad infinitum, nor the terms we use be empty of meaning. For the primary signification of every term employed must be clearly seen, and ought to need no proving 3 ; this being necessary, if we are to have something to which the point at issue or the problem or the opinion before us can be referred. ° Epicurus explains this more fully in Fr. 258 (Usener, p. 189). For " proof " and " proving " Bignone substitutes " declaration " and " declare."