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Chapter IX: What Essences can and what cannot be provedEdit

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. The arithmetician proceeds in this way, assuming both the nature of unity and also that it exists. In the case of things which have a middle term, things the cause of whose substance is something outside themselves, there may be a demonstration, as we have said, this cause being taken as a middle term, although the underlying essence of the thing is not demonstrated.