g. a whole group might be proved
through 'reciprocal replacement'-and of these one class are
identical in genus, namely all those whose difference consists in
their concerning different subjects or in their mode of manifestation.
This latter class may be exemplified by the questions as to the causes
respectively of echo, of reflection, and of the rainbow: the
connexions to be proved which these questions embody are identical
generically, because all three are forms of repercussion; but
specifically they are different.
Other connexions that require proof only differ in that the 'middle'
of the one is subordinate to the 'middle' of the other. For example:
Why does the Nile rise towards the end of the month? Because towards
its close the month is more stormy. Why is the month more stormy
towards its close? Because the moon is waning. Here the one cause is
subordinate to the other.
16
The question might be raised with regard to cause and effect whether
when the effect is present the cause also is present; whether, for
instance, if a plant sheds its leaves or the moon is eclipsed, there
is present also the cause of the eclipse or of the fall of the
leaves-the possession of broad leaves, let us say, in the latter case,
in the former the earth's interposition.
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