An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

244 THE GENERAL NATURE OF INDUCTION.
weigh the points of resemblance, not simply count them.
For a like reason we must weigh the points of difference,
and see whether the two cases differ in any fundamental
quality. The resemblances must be essential, the differ
ences unessential. General experience, and systematic
knowledge of the subject to which the given analogy
belongs, are the only means of distinguishing the
essential and the unessential.
The following example has been frequently used as an
illustration of Analogy. Prof. Minto quotes it from Reid
observe a
(Intellectual Powers, Essay I. ch. iii.) : "We may
very great similitude between this earth which we inhabit
and the other planets. They all revolve round the sun, as
the earth does, though at different distances and in different
periods. They borrow all their light from the sun, as the
earth does. Several of them are known to revolve round
their axis like the earth, and by that means have like suc
cession of day and night. Some of them have moons, that
serve to give them light in the absence of the sun, as our
moon does to us. They are all, in their motions, subject to
the same law of gravitation as the earth is. From all this
similitude it is not unreasonable to think that these planets
may, like our earth, be the habitation of various orders of
living creatures." The inference, as Reid states it, is, how
ever, defective in two ways, (i) Though all the points
which he mentions are important, he does not mention the
absolutely necessary conditions for the existence of life ; (2)
he neglects the possibility that the other planets may differ
from the earth in such ways that those essential conditions
are not fulfilled. What are the essential conditions of the
possibility of
1
life ? "By life we mean the existence of
organisms which depend upon the possession of a nitrog
enous compound, protoplasm, for the chemical changes
by which the phenomena of living are exhibited and ; upon
the presence in the atmosphere, or dissolved in water, of the
element oxygen, with which their nitrogenous constituents
1 That is, of "life" in the only
can conceive.
sense of the word which we