394 INDUCTION. 



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 properties, that it does so in a cer- 

 tain other property. The difference is, that in the case of a complete in- 

 duction it has been previously shown, by due comparison of instances, that 

 there is an invariable conjunction between the former property or proper- 

 ties and the latter property ; but in what is called analogical reasoning, no 

 such conjunction has been made out. There have been no opportunities of 

 putting in practice the Method of Difference, or even the Method of Agree- 

 ment; but we conclude (and that is all which the argument of analogy 

 amounts to) that a fact in, known to be true of A, is more likely to be true 

 of B if B agrees with A in some of its properties (even though no connec- 

 tion is known to exist between m and those properties), than if no resem- 

 blance at all could be traced between B and any other thing known to pos- 

 sess the attribute m. 



To this argument it is of course requisite that the properties common to 

 A with B shall be merely not 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 deduction from previous knowledge of the laws of the 

 properties in question, it can be concluded that they have nothing to do 

 with m, the argument of analogy is put out of court. The supposition 

 must be that m is an effect really dependent on some property of A, but 

 we know not on which. We can not point out any of the properties of A, 

 which is the cause of m, 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 possesses 

 one or more. This, accordingly, we consider as affording grounds, of more 

 or less strength, for concluding by analogy that B possesses the attribute m. 



There can be no doubt that everv such resemblance which can be point- 

 ed out between B and A, affords some degree of probability, beyond what 

 would otherwise exist, in favor of the conclusion drawn from it. If B re- 

 sembled A in all its ultimate properties, its possessing the attribute ra 

 would be a certainty, not a probability ; and every resemblance which can be 

 shown to exist between them, places it by so much the neai-er to that point. 

 If the resemblance be in an ultimate property, there will be resemblance in 

 all the derivative properties dependent on that ultimate property, and of 

 these m may be one. If the resemblance be in a derivative property, thei*e 

 is reason to expect resemblance in the ultimate property on which it de- 

 pends, 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 resemblances; the particular 

 I'esemblance sought will, therefore, be oftener found among things thus 

 known to resemble, than among things between which we know of no re- 

 semblance. 



For example, I might infer that there are probably inhabitants in the 

 moon, because there are inhabitants on the earth, in the sea, and in the air : 

 and this is the evidence of analogy. The circumstance of having inhabit- 

 ants is here assumed not to be an ultimate property, but (as is reasonable 

 to suppose) a consequence of other properties ; and depending, therefore, 

 in the case of the earth, on some of its properties as a portion of the uni- 

 verse, but on which of those properties we know not. Now the moon re- 



