ANALOGY. 333 



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 here nothing by 

 which to discriminate analogy from induction, since this type will sei-ve 

 for all reasoning from experience. In the most rigid induction, equally 

 with the faintest analogy, we conclude because A resembles B in one 

 or more properties, that it does so in a certain other property. The 

 difierence is, that in the case of a real induction it has been previously 

 shown, by due comparison of instances, that there is an invariable 

 conjunction between the former property or properties and the latter 

 property : but in what is called analogical reasoning, no such conjunc- 

 tion has been made out. There have been no opportunities of putting 

 in practice the Method of Difierence, or even the Method of Agree- 

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

 amounts to) that a fact 7n, 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 connexion 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 tliis argument it is of course requisite, that the properties com- 

 mon to A with B shall be merely not known to be connected wdth m; 

 they must not be properties knowm 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 con- 

 cluded that they have nothing to do with in, the argument of analogy 

 is put out of court. The supposition must be, that 7n is an effect, 

 really dependent upon some property of A, but we know not upon 

 which. We cannot 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 weight, for concluding by analogy that B 

 possesses the attribute m. 



There can be no doubt that every such resemblance which can be 

 pointed 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 resembled A in all its ultimate properties, its possessing 

 the attribute m would be a certainty, not a probability : and every re- 

 semblance 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 properties 

 dependent on that ultimate property, and of these vi may 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 upon the same ultimate property. 

 Every resemblance which can be shown to exist, affords gi-ound for 

 expecting an indefinite number of other resemblances ; the paiticular 

 resemblance sought will, therefore, be oftener found among things 

 thus known to resemble, than among things between which we know 

 of no resemblance.* 



♦ There was no greater foundation than this for Newton's celebrated conjecture that the 

 diamond was combustible. He grounded his guess upon the very high refractnig power of 

 the diamond, comparatively to its density ; a peculiarity which had been observed to exist 



