Aristotle - The Organon - index for ANALYTICA POSTERIORA Book 1 Part 9

Attribute demonstration only from 'appropriate' basic truths

Paragraph 1	It is clear that if the conclusion is to show an attribute inhering as such, nothing can be demonstrated except from its 'appropriate' basic truths.
Paragraph 2	Our knowledge of any attribute's connexion with a subject is accidental unless we know that connexion through the middle term in virtue of which it inheres, and as an inference from basic premisses essential and 'appropriate' to the subject - unless we know, e.g. the property of possessing angles equal to two right angles as belonging to that subject in which it inheres essentially, and as inferred from basic premisses essential and 'appropriate' to that subject:
Paragraph 3	It is no less evident that the peculiar basic truths of each inhering attribute are indemonstrable;
Paragraph 4	It is hard to be sure whether one knows or not;