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 moral nature like our own inhabits the universe." A tremendous leap—a leap from the physical to the moral. We know that these earth elements are found in the stars by actual observation and experience; but a moral nature like our own—this is assumed, and is not supported by the analogy.
John Burroughs: Analogy, True and False.
Notice the use of analogy in the argument below.
There is only one cure for the evils which newly acquired freedom produces; and that cure is freedom. When a prisoner first leaves his cell he cannot bear the light of day: he is unable to discriminate colors, or recognize faces. But the remedy is, not to remand him into his dungeon, but to accustom him to the rays of the sun. The blaze of truth and liberty may at first dazzle and bewilder nations which have become blind in the house of bondage. But, let them gaze on, and they will soon be able to bear it. In a few years men learn to reason. The extreme violence of opinions subsides. Hostile theories correct each other. The scattered elements of truth cease to contend and begin to coalesce, and at length a system of justice and order is educed out of the chaos.
—Macaulay: Milton.
+193. Summary of Arrangement.+—The necessity of argument arises because some one does not believe the truth of a proposition. To establish in his mind a belief, we must present our arguments in an orderly and convincing way. The order will usually be to show him first the possibility and then the probability, and finally to lead him as near to certainty as we can. We may say, therefore, that we should use arguments from cause, arguments from sign, and arguments from example in the order named.