Aristotle

“ 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. ”