After stating some other additional difficulties which seem to leave the work of Definition inexplicable, Aristotle relinquishes the dubitative treatment, and looks out for some solution of the puzzle: How may it be possible that the Definition shall become known?[17] He has already told us that to know the essence of a thing is the same as to know the cause or reason of its existence; but we must first begin by knowing that the _definiendum_ exists; for there can be no definition of a non-entity, except a mere definition of the word, a nominal or verbal definition. Now sometimes we know the existence of the subject by one or other of its accidental attributes; but this gives us no help towards finding the definition.[18] Sometimes, however, we obtain a partial knowledge of its essence along with the knowledge of its existence; when we know it along with some constant antecedent, or through some constant, though derivative, consequent. Knowing thus much, we can often discover the cause or fundamental condition thereof, which is the essence or definition of the subject.[19] Indeed, it may happen that the constant derivative, and the fundamental essence on which it depends, become known both together; or, again, the cause or fundamental condition may perhaps not be the essence of the subject alone, but some fact including other subjects also; and this fact may then be stated as a middle term. Thus, in regard to eclipse of the moon, we know the constant phenomenal fact about it, that, on a certain recurrence of the time of full moon, the moon casts no light and makes no shadow. Hence we proceed to search out the cause. Is it interposition of the earth, or conversion of the moon's body, or extinction of her light, &c.? The new fact when shown, must appear as a middle term, throwing into syllogistic form (in the First figure) the cause or rational explanation of a lunar eclipse; showing not merely that there is an eclipse, but what an eclipse is, or what is its definition.[20] [Footnote 17: Analyt. Post. II. vii. p. 92, a. 34, seq. The [Greek: a)po/riai] continue to the end of ch. vii. He goes on, ch. viii. p. 93, a. 1-2: [Greek: pa/lin de\ skepte/on ti/ tou/tôn le/getai kalô=s, kai\ ti/ ou) kalô=s], &c. "Tout ce qui précède ne représente pas la théorie proprement dite; ce n'est qu'une discussion préliminaire" (Barth. St. Hilaire, not. p. 222). These difficult chapters are well illustrated by Hermann Rassow, ch, i. pp. 9-14.]