e. either
to all species of plant or to a single species. So in these
universal and commensurate connexions the 'middle' and its effect must
reciprocate, i.e. be convertible. Supposing, for example, that the
reason why trees are deciduous is the coagulation of sap, then if a
tree is deciduous, coagulation must be present, and if coagulation
is present-not in any subject but in a tree-then that tree must be
deciduous.
17
Can the cause of an identical effect be not identical in every
instance of the effect but different? Or is that impossible? Perhaps
it is impossible if the effect is demonstrated as essential and not as
inhering in virtue of a symptom or an accident-because the middle is
then the definition of the major term-though possible if the
demonstration is not essential. Now it is possible to consider the
effect and its subject as an accidental conjunction, though such
conjunctions would not be regarded as connexions demanding
scientific proof. But if they are accepted as such, the middle will
correspond to the extremes, and be equivocal if they are equivocal,
generically one if they are generically one.
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