Aristotle

“ What Essences can and what cannot be proved When the Cause of the thing and the thing itself are identical, no demonstration is possible; for we have then reached ultimate principles, the nature of which must be assumed. When the Cause is outside the thing the Cause may be used as a middle term in the demonstration. Some things have, others have not a cause outside themselves. It is clear therefore that of a thing’s essential qualities some are ultimate and primary, the existence and nature of which must either be assumed by hypothesis or made clear in some other way than by demonstration. ”