Front Page Titles (by Subject) CHAP. VII.: Whether the Essence can in any way be proved - Posterior Analytics
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CHAP. VII.: Whether the Essence can in any way be proved - Aristotle, Posterior Analytics 
Aristotle’s Posterior Analytics, trans. E.S. Bouchier, B.A. (Oxford: Blackwell, 1901).
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Whether the Essence can in any way be proved
Definition does not prove the essence of things, for, if it proved a thing’s essential nature, it would also prove that the thing exists. It does not however merely explain the meaning of words, for then every word we uttered would be a definition. The objects of definition and demonstration are entirely different, and neither gives knowledge of a thing’s essence.
How then is definition to prove the Substance or Nature of a subject? It will neither shew, as if demonstrating from admitted premises, that, when certain premises exist, something else must necessarily follow, nor will it shew, as induction shews from the evidence of particular instances, that everything must have a certain quality because nothing is without that quality; for induction does not prove the nature of a thing but only that the thing does or does not exist. What other method of demonstration then remains? It is surely impossible to demonstrate by an appeal to the senses, or by pointing at a thing with the finger.
Further, how is one to prove the essential nature of a thing? It is necessary, in order to learn the nature of Man or of anything else, to know that the subject in question exists; for no one can know the nature of the non-existent, but only what its name or other designation signifies, as when I say e.g. ‘Goatstag’; for the nature of a goatstag it is impossible to know. Moreover, though one may prove both the nature of a thing and that it exists, how can it be possible to do so by one and the same method? Definition and demonstration each prove one single thing; but ‘what man is,’ and ‘that man exists’ are two different questions.
Further we hold that the existence of anything must needs be proved by demonstration, with the exception of the essence. Existence as such is in no case identical with the essence, for ‘that which exists’ does not form a separate genus. Hence demonstration will only prove that the thing exists, not what it is. This may be seen in the separate sciences. E.g. the geometrician assumes the meaning of the word Triangle, but proves that Triangle exists. What is it then which definition will prove? Will it be the essence of triangle? In that case one would know by definition the essential nature of triangle without knowing whether triangle exists. This is impossible. It is also clear from the present system of definition that definitions do not prove the existence of a thing. Even if according to the definition, lines drawn from the centre to the circumference of the circle are equal, what proves the existence of lines or circle? Why is the thing defined a circle and nothing else? Why might one not call it bronze just as well as circle? If then definition must prove either the nature of the thing or the meaning of the name, if further the former be impossible, a definition would appear to be a phrase with the same meaning as the subject itself. This is untenable, for in the first place there would be definitions of things which are not substances and of things which do not exist at all, for even non-existing things may be denoted by a name. Further every phrase would in that case be a definition, for it is possible by means of a word to impose any name whatever on a thing, so that all of us would be talking in definitions, and the name Iliad would be a definition. Also no science could demonstrate that a particular name denoted a particular thing. Hence definition and syllogism are clearly not identical, and have not identical objects. Further definition does not demonstrate or prove anything, nor can one know the essential nature of a thing either by definition or demonstration.