THE MARK OF SIMILARITY.
As opposed to analogy by type there is a second form; namely, analogy by one or more similar marks or qualities. This form is best described by the definition: When two things resemble each other in a few marks or qualities they resemble each other in other marks or qualities.
Illustrations of analogy by marks.
(1) Noting that two students have the same surname, I infer that they are brothers.
(2) A man with a book under his arm rings the door bell and asks to see “the lady of the house.” At once the conclusion is drawn that the caller is a book agent.
(3) Two automobiles, resembling each other in shape of body, force one to the conclusion that the machines are of the same make.
THE ERRORS OF ANALOGY BY MARKS OF SIMILARITY.
It follows that analogy by example gives generalizations of much greater certitude than analogy by one or two marks of resemblance. Here is a field bespattered from boundary to boundary with erroneous thinking. The principle of resemblance being an innate tendency, this form of error is most common with the immature. The child reasons by analogy when he invests the poodle with the despised cognomen of “kitty”; or honors every man who wears glasses with “papa.” In the childhood of the race natural events were interpreted by means of analogy. The wind blowing through the trees made sounds much like the human voice; hence these noises were attributed to spirits. Primeval man was led to believe by analogy that everything which moved was alive. We may, therefore, think of our revered forbear as engaged in the undignified task of running after his shadow, or chasing a leaf around a stump.
THE VALUE OF ANALOGY.
Analogy being rich in its suggestions is the favored process of the scientist and inventor. Newton reasoned by analogy when he tentatively affirmed of the moon what he positively knew of the apple. Franklin’s reasoning was analogical when he discovered the identity of the electric spark and lightning. Because this form of induction so often leads to error and at best involves a degree of probability far below induction by analysis, some logicians are inclined to ignore its generalizations altogether. Others deem this a mistake because of these reasons: First. Analogy is serviceable to a high degree in suggesting hypotheses which may be advanced either for the purpose of explanation or verification. It has already been indicated that analogy is the common instrument used by the inventor and discoverer. Second. The principle of analogy, in reality, lies at the basis of classification; because in this, things are grouped according to their resemblances. Third. Analogical induction affords valuable training in originality and initiative. A mind which easily and naturally discerns analogies is “fertile in new ideas.”