792 ANALOGY agreeing with one another in certain respects agree also in certain other respects, yet differ in respect of their degree of evidence. In both the argument is from known points of agreement to unknown ; but, whereas in induction the known points of agreement are supposed by due compari son of instances to have been ascertained as the material ones for the case in hand or conclusion in view, in other words, to be invariably connected by way of causation with the inferred properties, it is otherwise in analogy, where it is only supposed that there is no incompatibility between the inferred properties and the common properties, or known points of resemblance, that are taken as the ground of inference. Thus, if by comparison of instances it had been ascertained, or otherwise it were known, that organic life is dependent on the bare possession of an atmosphere in planetary bodies rotating upon an axis, then it would be an induction to infer the presence of life upon any heavenly body, known or as yet undiscovered, in which these conditions should be detected. With our actual knowledge, confined to the case of the Earth, and only enabling us to say that the absence of an atmosphere must destroy life, the inference to such a planet as Mars, where the conditions stated seem to be present, is but analogical ; while to the Moon, which seems to have no atmosphere, the inference has not even this amount of force, but there is rather ground for inductively concluding against the possibility of organic life. Upon this view it ceases to be characteristic of analogy that the inference should be to a particular case only; for the inductive conclusion, when the evidence is of a kind to admit of such being drawn, may as well be particular; and, again, it may equally well happen that the analogical inference, where nothing stronger can. be drawn, should have universal application. Notwithstanding, it will be found in general that, where the evidence, consisting of bare similarity of attributes in two or more particular instances, permits only of an ana logical inference being made, the extension in thought takes place to particular cases only which have a special interest, and the mind hesitates to commit itself to a general law or rule. Mill, therefore, though he does not raise the point, is practically at one with Aristotle and all others who make example or analogy to consist in the passage from one or more particular cases to a particular .new case bearing resemblance to the former. It is his peculiar merit to have determined the specific conditions under which the passage in thought, whether to a particular or a general, acquires the authority of an effective induction. Analogy is so much resorted to in science in default of induction, either provisionally till induction can be made, or as its substitute where the appropriate evidence cannot be obtained, it is also much relied upon in practical life for the guidance of conduct, that it becomes a matter of great importance to determine its conditions. Whether in science or in the affairs of life, the abuse of the process, or what. is technically called False Analogy, is one of the most besetting snares set for the human mind. It is obvious that, as the argument from analogy proceeds upon bare resemblance, its strength increases with the amount of similarity ; so that, though no connection is, or can be, in ductively made out between any of the agreeing properties and the additional property which is the subject of infer ence, yet (in Mill s words), " where the resemblance is very great, the ascertained difference very small, and our know ledge of the subject-matter very extensive, the argument from analogy may approach in strength very near to a valid induction. If (he continues), after much observa tion of B, we find that it agrees with A in nine out of ten of its known properties, we may conclude, with a pro bability of nine to one, that it will possess any given deriva tive property of A" (Logic, b. iii., c. xx., 3). But it is equally obvious that against the resemblances the ascertain- able differences should be told off. For bare analogy, the differences in the two (or more) cases must as little as the resemblances be known to have any connection, one way or the other, with the point in question ; both alike must only not be known to be immaterial, else they should fall quite out of the reckoning. As regards the differences, however, this is what can least easily be discovered, or is, by the mind in its eagerness to bring things together, most easily overlooked ; and, accordingly, the error of false analogy arises chiefly from neglecting so to con sider them. Thus, if the inference is to the presence of organic life of the terrestrial type on other planetary bodies, any agreements, even when extending to the details of chemical constitution, are of small account in the positive sense, compared with the negative import of such facts as absence of atmosphere in the Moon, and excess of heat or cold in the inmost or outermost planets. To neglect such points will not simply make the analogy loose ; but, as the very point in question is concerned in them, the analogy becomes false and positively misleading. Still greater is the danger when the things analogically brought together belong not at all to the same natural classes, but the resemblance is only in some internal relation of each to another thing of its own kind; as when, for example, under the name of motives, particular states of mind (feelings, &c.) are supposed to determine the action of a man, as tho motion of a body may be determined by a composition of forces. In such cases there may be nothing to prevent the drawing of a good analogy upon a strictly limited issue ; nay, there may even sometimes, in special circumstances, be ground Tor drawing an inductive conclusion ; but gene rally the elements of difference are so numerous, and their import either so hard to appreciate, or, when appreci able, so decisive in a sense opposite to the conclusion aimed at, that to leave them out of sight and argue with out reference to them, as the mind is tempted to do, vitiates the whole proceeding. What is not" sufficient for analogy may, however, be good as metaphor, and metaphor is of no small use for expository purposes; while (as Mill says), though it is not an argument, it may imply that an argu ment exists. The sense just mentioned of a resemblance of relations suggests the question how far the common argument from analogy and mathematically determinate proportion, which was originally called by the name, are cognate processes. Undoubtedly the common argument, proceeding upon resemblance in the properties of things, can be made to assume roughly the guise of a proportion, e.g., Earth : Mars : : Men : Mars -dwellers, or Earth : Men = Mars : Mars- dwellers, the-fact of planetary nature, or other resembling attributes gone upon, being regarded as common exponent. Less easy is it to interpret a determinate proportion, with numerical equality of ratios, as analogy in the common sense ; for here the very determinateness makes all the difference. The name analogy is so suggestive to English readers of Bishop Butler s famous treatise, that a word, in conclusion, seems called for on the nature and scope of the particular application of the process made by him. His work is entitled Tfie Analogy of Religion, Natural and Revealed, to the Constitution and Course of Nature, and consists in an attempt to convince deists that there are no difficulties urged against revelation, or the system of natural religion, which do not bear with equal force against the order of nature as determined by Providence. The argument is a perfectly fair one within the limits assigned, and Butlei must be allowed the credit of very well apprehending the logical conditions involved in it. In his introduction he
understates rather than overstates the strength of his posi-
