in Philosophy, a certain relation and agreement between two or more things, which in other respects are entirely different.
There is likewise an analogy between things that have some conformity or resemblance to one another; for example, between animals and plants; but the analogy is still stronger between two different species of certain animals.
Analogy enters much into all our reasoning, and serves to explain and illustrate. A great part of our philosophy, indeed, has no other foundation than analogy.
It is natural to mankind to judge of things less known, by some similitude, real or imaginary, between them and things more familiar or better known. And where the things compared have really a great similitude in their nature, when there is reason to think that they are subject to the same laws, there may be a considerable degree of probability in conclusions drawn from analogy. Thus we may observe a very great similitude between this earth which we inhabit, and the other planets, Saturn, Jupiter, Mars, Venus, and Mercury. They all revolve round the sun, as the earth does, although 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, must have a like succession 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 those planets may, like our earth, be the habitation of various orders of living creatures. There is some probability in this conclusion from analogy.
But it ought to be observed, that, as this kind of reasoning reasoning can afford only probable evidence at best; so, unless great caution be used, we are apt to be led into error by it. To give an instance of this: Anatomists, in ancient ages, seldom dissected human bodies; but very often the bodies of those quadrupeds whose internal structure was thought to approach nearest to that of the human body. Modern anatomists have discovered many mistakes the ancients were led into, by their conceiving a greater similitude between the structure of men and of some beasts than there is in reality.
Perhaps no author has made a more just and a more happy use of this mode of reasoning, than Bishop Butler in his Analogy of Religion, Natural and Revealed, to the Constitution and Course of Nature. In that excellent work, the author does not ground any of the truths of religion upon analogy, as their proper evidence. He only makes use of analogy to answer objections against them. When objections are made against the truths of religion, which may be made with equal strength against what we know to be true in the course of nature, such objections can have no weight.
Analogue reasoning, therefore, may be of excellent use in answering objections against truths which have other evidence. It may likewise give a greater or a less degree of probability in cases where we can find no other evidence. But all arguments drawn from analogy are still the weaker, the greater disparity there is between the things compared; and therefore must be weakest of all when we compare body with mind, because there are no two things in nature more unlike.
There is no subject in which men have always been so prone to form their notions by analogies of this kind, as in what relates to the mind. We form an early acquaintance with material things by means of our senses, and are bred up in a constant familiarity with them. Hence we are apt to measure all things by them; and to ascribe to things most remote from matter the qualities that belong to material things. It is for this reason that mankind have, in all ages, been so prone to conceive the mind itself to be some subtle kind of matter; that they have been disposed to ascribe human figure and human organs not only to angels, but even to the Deity.
To illustrate more fully that analogue reasoning from a supposed similitude of mind to body, which appears to be the most fruitful source of error with regard to the operations of our minds, the following instance may be given. When a man is urged by contrary motives, those on one hand inciting him to do some action, those on the other to forbear it; he deliberates about it, and at last resolves to do it, or not to do it. The contrary motives are here compared to the weights in the opposite scales of a balance; and there is not perhaps any instance that can be named of a more striking analogy between body and mind. Hence the phrase of weighing motives, of deliberating upon actions, are common to all languages.
From this analogy, some philosophers draw very important conclusions. They say, that as the balance cannot incline to one side more than the other when the opposite weights are equal, so a man cannot possibly determine himself if the motives on both hands are equal; and as the balance must necessarily turn to that side which has most weight, so the man must necessarily be determined to that hand where the motive is strongest. And on this foundation some of the schoolmen maintained, that if a hungry ass were placed between two bundles of hay equally inviting, the beast must stand still and starve to death, being unable to turn to either, because there are equal motives to both. This is an instance of that analogue reasoning, which, it is conceived, ought never to be trusted; for the analogy between a balance and a man deliberating, though one of the strongest that can be found between matter and mind, is too weak to support any argument. A piece of dead inactive matter, and an active intelligent being, are things very unlike; and because the one would remain at rest in a certain case, it does not follow that the other would be inactive in a case somewhat similar. The argument is no better than this, that because a dead animal moves only as it is pushed, and if pushed with equal force in contrary directions, must remain at rest; therefore the same thing must happen to a living animal: for surely the similitude between a dead animal and a living is as great as that between a balance and a man.
The derivation of the word Analogy indicates, as Professor Caillillon of Berlin * observes, a resemblance discernible by reason. This is confirmed by the sense in which the term is used in geometry, where it signifies an equality of ratios. In explaining this subject, it is observed, there may be a resemblance between sensations and a resemblance between perceptions: the former is called physical resemblance, because it acts upon the physical or sensitive faculty; the latter moral resemblance, because it affects the moral or rational faculty of man.
Every resemblance may be reduced to an equality in sensations or perceptions; but this supposes some equality in their causes: we say some equality, because the disposition of the organs, or of the soul, must necessarily affect the sensations or perceptions; but this can influence only their degree, and not their nature.
The character of one person resembles that of another only when they both speak and act so as to excite equal perceptions, or, to speak more strictly, the same perception; when they both display vivacity or indifference, anger or meekness, on the same occasions, and both excite in the soul of the observer identical perceptions, or rather the same perception of vivacity or indifference, of anger or meekness. These identical perceptions, the degree of which will depend much on the disposition of the observer's mind, must have identical causes, or, in other words, the same cause; which is the vivacity or indifference, the anger or meekness, displayed by each of these characters.
Every physical resemblance may therefore be reduced to one or more equalities, and every moral resemblance to one or more identities. Wherever there is moral resemblance there is analogy. Analogy may therefore be reduced to identity, and always supposes comparison.
Two objects are said to have an analogy to each other, or are called analogous, when some identity is discovered upon comparing them. An analogue conclusion is a conclusion deduced from some identity.
The principles of analogy are a comparison of two objects; and one or more identities resulting from their being thus compared. The characters of analogy are—two objects be compared—that there be one or more identities between these objects—and that this is discernible only by reason or intellect. Physical resemblance is to the senses what analogy is to the understanding. The former, when perfect, becomes equality; the latter, identity.
Resemblance and analogy are the foundations both of probability and of certainty. When we are not satisfied that the resemblance or the analogy is complete, we stop at probability; which becomes certainty when we are, or think we are, assured that the resemblance or the analogy is perfect.
In reasoning by analogy, we should be careful not to confound it with resemblance; and also not to deduce from the identity or identities, on which the analogy is founded, a conclusion which has either no relation, or only a partial relation, to these identities.
The principal uses of analogy in the investigation of physical and moral truth, according to our author, may be reduced to the four following: 1. By means of our senses to improve, first our own judgment, and afterwards that of others, with respect to intellectual subjects. 2. To deduce a general from a particular truth. Having discovered and proved the truth of a proposition with respect to any particular object, examine whether this truth flows from a quality peculiar to this single object, or common to several objects. In the latter case all these objects may be comprehended under one general idea, founded on their common quality. Substitute this general idea instead of the particular object, and the proposition will become general, without ceasing to be true; because whatever evidently and solely results from the identity on which an analogy is founded, must necessarily be true with respect to all those objects in which the analogy is the same. 3. To prove the truth or falsehood of propositions which cannot be otherwise demonstrated. 4. To discover new truths in both natural and moral philosophy.
Analogy, among grammarians, is the correspondence which a word or phrase bears to the genius and received forms of any language.