Posterior Analytics (Bouchier)/Book II/Chapter XV
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Book II, Chapter XV
Chapter XV: How far the same Middle Term is employed for demonstrating different Questions 
- Questions for demonstration are the same when they use the same middle term. Questions may be generically the same and specifically different.
Questions for solution are the same, first from having the same middle term (as for instance all questions which can be solved by the common middle term ‘reactionary influence,’) and of these some are generically identical while possessing certain specific differences, whether of object or only of method. Take the three questions ‘What produces an Echo?’ ‘Why are objects reflected?’ ‘What causes a Rainbow?’ All these are generically one, for all involve refraction, but they differ specifically. In other cases questions differ in that the middle term of the one is subordinate to that of the other. Thus, ‘Why is the current of the Nile stronger at the end of the month?’
‘Because the end of the month is more rainy.’
‘Why then is the end of the month more rainy?’
‘Because the moon is waning.’
These two questions stand to one another in the second of the above relations.