Analogy, a term, originally Greek, which signifies an agreement or correspondence in certain respects between things in other respects different. It makes a resemblance of relations, as in the phrase, 'Knowledge is to the mind what light is to the eye.' Euclid employed it to signify proportion, or the equality of ratios, and it has retained this sense in mathematics; but it is a term little used in the exact sciences, and of very frequent use in every other department of knowledge and of human affairs. In Grammar, we speak of the analogy of language—i.e. the correspondence of a word or phrase with the genius of the language, as learned from the manner in which its words and phrases are ordinarily formed. Analogy, in fact, supposes a rule inferred from observation of instances, and upon the application of which, in other instances not precisely, but in some respects, similar, we venture, with more or less confidence, according to the degree of ascertained similarity, and according to the extent of observation from which our knowledge of the rule has been derived. The opposite to analogy is anomaly (Gr., 'irregularity'); and this term is used not only in grammar, but with reference to objects of natural history which in any respect are exceptions to the ordinary rule of their class or kind. Here it strictly means the resemblance of function between organs which are essentially different (see ANALOGUE above).
In the progress of science, analogies have been discovered pervading all nature, and upon which conclusions are often based with great confidence and safety. It is a kind of presumptive reasoning from parallel cases that indeed warrants only probable conclusions; but the probability may become of a very high degree, and in the affairs of life we must often act upon conclusions thus attained. Reasoning from analogy, however, requires much caution in the reasoner. Yet even when its conclusions are very uncertain, they often serve to guide inquiry and lead to discovery. Many of the most brilliant discoveries recently made in natural science were the result of investigations thus directed. Where the proper evidence of truth is of another kind, arguments from analogy are often of great use for the removal of objections. It is thus that they are employed by Bishop Butler in his Analogy of Religion, Natural and Revealed, to the Constitution and Course of Nature. In law, reasoning from analogy must often, to a certain extent, be admitted in the application of statutes to particular cases. Upon similar reasoning the practice of medicine very much depends. To discover the meaning of any literary work, it is also often necessary; the sense of the author in a passage somewhat obscure being in some measure determined according to passages in which he has expressed himself more clearly. The application of this rule to the interpretation of Scripture is a point of difference between Protestants and Roman Catholics, the latter insisting upon the interpretation of difficult passages solely by ecclesiastical tradition and authority, while the former claim the right to apply analogy of interpretation. The inspiration of Scripture, however, when fully admitted, warrants a more confident use of analogical reasoning than in the case of the works of an uninspired author.