Posterior Analytics (Bouchier)/Book II/Chapter II
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Book II, Chapter II
Chapter II: Every question is concerned with the discovery of a Middle Term 
- The first and third of these questions and also the second and fourth may be identified. Hence all scientific enquiry consists in investigating whether there is a middle term, and what the middle term is, for the middle is identical with the cause.
These or such as these are the subjects about which we enquire and which we know when we have found what we sought. Now when we ask about the fact, or enquire whether the thing has absolute existence, we enquire whether it has a middle term, but when we have learned the fact and solved the question as to its absolute or partial existence, then we ask what the middle term is. My phrase ‘partial existence’ would be illustrated by the questions ‘Does the moon wax?’ or ‘Is the moon eclipsed?’ In questions of this sort we do really ask whether a thing exists or not. ‘Absolute existence’ might be illustrated by the questions ‘Does a moon, or does night, exist or not?’
Hence it follows that in every enquiry we really ask if a middle term for the subject in question is or else what this middle term is. The reason is that the middle term contains the cause, and it is the cause that we look for in all cases. For instance we ask first ‘Is the moon eclipsed?’ Then, ‘Is there any cause of the eclipse or not?’ Next, on learning that some cause of it is known we enquire what the cause is. Now the cause of a thing’s being, (not of its being this or that, but of its being absolutely) or again the cause why a thing has no absolute existence but is an essential or accidental attribute of something else, is nothing but the middle term. When speaking of absolute existence I refer to the existence of the subject, whether it be moon, earth, sun, or triangle; examples of attributes would be eclipse, equality, inequality, interposition or non-interposition of the earth.
In all these cases it is clear that the nature of the thing and its cause are the same. To the question ‘What is an eclipse?’ the answer is ‘An exclusion of light from the moon owing to the interposition of the earth.’ ‘Why does an eclipse take place, or why is the moon eclipsed?’ ‘Because the light fails when the earth excludes it.’ ‘What is harmony?’ ‘An arithmetical proportion between sharp and flat.’ ‘Why does sharp harmonize with flat?’ ‘Because they are in a certain arithmetical proportion.’ Thus the question ‘Can sharp and flat harmonize?’ is equivalent to ‘Is there an arithmetical proportion between them?’ On learning that there is we proceed to ask, ‘What then is the proportion?’ That the object of our enquiry is really the middle term is clearly displayed by those cases in which the middle term is perceptible to the senses. We make an enquiry about it only when we have not perceived it. Thus, in the case of eclipse, we ask whether there is such a thing or not. If, however, we were on the moon we should not enquire whether an eclipse does occur, nor yet why it occurs, for the answers to both these questions would become visible simultaneously. We should in fact have learned the universal as a result of sense perception. Sense perception would shew that the earth was at a particular moment excluding the sun’s light; and since it would also be obvious that the moon was then being eclipsed, knowledge of the universal would have been attained immediately. Thus, as we have said, knowing the nature of a thing is the same as knowing its cause. The former of these may either have or not have an independent existence. E.g. ‘One thing is larger, or smaller, than another.’ ‘The three angles of a triangle are equal to two right angles.’
It has now been made clear that every kind of enquiry involves a search after the middle term.