ANALOGY. 365 



analogy between the two cases ; a 

 mode of speech which implies that 

 when the analogy can be proved, the 

 argfument founded on it cannot be 

 resisted. 



§ 2. It is on the whole more usual, 

 however, to extend the name of ana- 

 logical evidence to arguments from 

 any sort of resemblance, provided 

 they do not amount to a complete 

 induction : without peculiarly dis- 

 tinguishing resemblance of relations. 

 Analogical reasoning, in this sense, 

 may be reduced to the following for- 

 mula : — Two things resemble each 

 other in one or more respects ; a 

 certain proposition is true of the one, 

 therefore it is true of the other. But 

 we have nothing here by which to 

 discriminate analogy from induction, 

 since this type will serve for all 

 reasoning from experience. In the 

 strictest induction, equally with the 

 faintest analogy, we conclude because 

 A resembles B in one or more pro- 

 perties, that it does so in a certain 

 other property. The difference is, 

 that in the case of a complete induc- 

 tion it has been previously shown, 

 by due comparison of instances, that 

 there is an invariable conjunction 

 between the former property or pro- 

 perties and the latter property ; but 

 in what is called analogical reasoning, 

 no such conjunction has been made 

 out. There have been no oppor- 

 tunities of putting in practice the 

 Method of Difference, or even the 

 Method of Agreement ; but we con- 

 clude (and that is all which the argu- 

 ment of analogy amounts to) that a 

 fact m, known to be true of A, is 

 more likely to be true of B if B 

 agrees with A in some of its proper- 

 ties, (even though no connection is 

 known to exist between m and those 

 properties,) than if no resemblance at 

 all could be traced between B and 

 any other thing known to possess the 

 attribute m. 



To this argument it is of course 

 requisite that the properties common 

 Xq a with B shall be merely pot 



known to be connected with m ; they 

 must not be properties known to be 

 unconnected with it. If, either by 

 processes of elimination, or by deduc- 

 tion from previoiis knowledge of the 

 laws of the properties in question, it 

 can be concluded that they have no- 

 thing to do with m, the argument of 

 analogy is put out of court. The 

 supposition must be that in is an 

 effect really dependent on some pro- 

 perty of A, but we know not on 

 which. We cannot point out any of 

 the properties of A which is the 

 cause of ?», or united with it by 

 any law. After rejecting all which 

 we know to have nothing to do with 

 it, there remain several between 

 which we are unable to decide : of 

 which remaining properties B pos- 

 sesses one or more. This accordingly 

 we consider as affording grounds, of 

 more or less strength, for concluding 

 by analogy that B possesses the attri- 

 bute m. 



There can be no doubt that every 

 such resemblance which can be 

 pointed out between B and A aflfords 

 some degree of probability, beyond 

 what would otherwise exist, in favour 

 of the conclusion drawn from it. If 

 B resembled A in all its ultimate 

 properties, its possessing the attribute 

 m would be a certainty, not a proba- 

 bility ; and every resemblance which 

 can be shown to exist between them 

 places it by so much the nearer to 

 that point. If the resemblance be in 

 an ultimate property, there will be 

 resemblance in all the derivative pro- 

 perties dependent on that ultimate 

 property, and of these m maj'^ be one. 

 If the resemblance be in a derivative 

 property, there is reason to expect 

 resemblance in the ultimate property 

 on which it depends, and in the other 

 derivative properties dependent on 

 the same ultimate property. Every 

 resemblance which can be shown to 

 exist affords ground for expecting an 

 indefinite number of other resem- 

 blances : the particular resemblance 

 sought will, therefore, be oftener 

 found among things thus known to 



