+192. Argument from Analogy.+--Analogy is very much relied upon in
practical life. Reasoning from analogy depends upon the recognition of
similarity in regard to some particulars followed by the inference that
the similarity extends to other particulars. As soon as it was known that
the atmospheric conditions of the planet Mars are similar to those of the
earth, it was argued by analogy that Mars must also, be inhabited.

An analogy is seldom conclusive and, though it is often effective in
argument, it must not be taken as proof of fact. The mind very readily
observes likenesses, and when directed toward the establishing of a
proposition easily overlooks the differences. In order to determine the
strength of an argument from analogy, attention should be given to the
differences existing between the two propositions considered. False
analogies are very common. We must guard against using them, and
especially against allowing ourselves to be convinced by them. Even when
the resemblance is so slight as to render analogy impossible, it may serve
to produce a metaphor that often has the effect of argument.

It is much easier to captivate the fancy with a pretty or striking figure
than to move the judgment with sound reason.... His (the speaker's)
picture appeals to the mind's visible sense, hence his power over us,
though his analogies are more apt to be false than true....

The use of metaphor, comparison, analogy, is twofold--to enliven and to
convince; to illustrate and enforce an accepted truth, and to press home
and clinch one in dispute. An apt figure may put a new face upon an old
and much worn truism, and a vital analogy may reach and move the reason.
Thus when Renan, referring to the decay of the old religious beliefs, says
that the people are no poorer for being robbed of false bank notes and
bogus shares, his comparison has a logical validity....

The accidental analogies or likenesses are limitless, and are the great
stock in trade of most writers and speakers. An ingenious mind finds types
everywhere, but real analogies are not so common. The likeness of one
thing to another may be valid and real, but the likeness of a thought with
a thing is often merely fanciful....

I recently have met with the same fallacy in a leading article in one of
the magazines. "The fact revealed by the spectroscope," says the writer,
"that the physical elements of the earth exist also in the stars, supports
the faith that a mo