Aristotle

“ How the Essence can be proved When a thing is once known its essence and its cause are identified. The essence cannot be demonstrated, but before seeking for the cause one must know that the thing exists. Hence, though demonstration cannot give the essence, without it the essence could never be learned. ”

“ As a second example, take the question ‘with which of these two contradictory majors does the definition of triangle agree:—a triangle has its angles equal to two right angles; no triangle has its angles equal to two right angles’? If the premises are immediate truths we learn the fact and the cause of the quality simultaneously. If no demonstration be possible we know the fact but not the cause. ”

“ We have then shewn how the nature of a thing is attained to and becomes known, and it follows that there can be no syllogism or demonstration proving a thing’s nature, though this may become clear as a result of syllogism and demonstration. Thus we cannot know a thing’s nature without demonstration, in cases where the cause is outside the thing ”