Aristotle - The Organon - index for ANALYTICA PRIORIA Book 1 Part 23

Every syllogism is formed through these figures

Paragraph 1 It is clear from what has been said that the syllogisms in these figures are made perfect by means of universal syllogisms in the first figure and are reduced to them.
Paragraph 2 It is necessary that every demonstration and every syllogism should prove either that something belongs or that it does not, and this either universally or in part, and further either ostensively or hypothetically.
Paragraph 3 If then one wants to prove syllogistically A of B, either as an attribute of it or as not an attribute of it, one must assert something of something else.
Paragraph 4 It is clear then that the ostensive syllogisms are effected by means of the aforesaid figures;

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