Aristotle - The Organon - index for ANALYTICA POSTERIORA Book 1 Part 4

Scientific knowledge must rest on essential premisses

Paragraph 1 Since the object of pure scientific knowledge cannot be other than it is, the truth obtained by demonstrative knowledge will be necessary.
Paragraph 2 Extending this classification to all other attributes, I distinguish those that answer the above description as belonging essentially to their respective subjects;
Paragraph 3 Further (a) that is essential which is not predicated of a subject other than itself:
Paragraph 4 In another sense again (b) a thing consequentially connected with anything is essential;
Paragraph 5 So far then as concerns the sphere of connexions scientifically known in the unqualified sense of that term, all attributes which (within that sphere) are essential either in the sense that their subjects are contained in them, or in the sense that they are contained in their subjects, are necessary as well as consequentially connected with their subjects.
Paragraph 6 Thus, then, we have established the distinction between the attribute which is 'true in every instance' and the 'essential' attribute.
Paragraph 7 I term 'commensurately universal' an attribute which belongs to every instance of its subject, and to every instance essentially and as such;
Paragraph 8 An attribute belongs commensurately and universally to a subject when it can be shown to belong to any random instance of that subject and when the subject is the first thing to which it can be shown to belong.

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