XXVIIL] ANALOGY. 641 



inhabited, but the absence of any appreciable atmosphere 

 renders the existence of inhabitants highly improbable. 

 I Speculations resting upon weak analogies hardly belong 

 to the scope of true science, and can only be tolerated as 

 an antidote to the far worse dogmas which assert that the 

 thousand million of persons on earth, or rather a small 

 fraction of them, are the sole objects of care of the Power 

 which designed this limitless Universe. 



Failures of Analogy. 



So constant is the aid which we derive from the use of 

 analogy in all attempts at discovery or explanation, that it 

 is most important to observe in what cases it may lead us 

 into difficulties. That which we expect by analogy to 

 exist 



(1) May be found to exist; 



(2) May seem not to exist, but nevertheless may really 

 exist ; 



(3) May actually be non-existent. 



In the second case the failure is only apparent, and 

 arises from our obtuseness of perception, the smallness of 

 the phenomenon to be noticed, or the disguised character 

 in which it appears. I have already pointed out that the 

 analogy of sound and light seems to fail because light does 

 not apparently bend round a corner, the fact being that 

 it does so bend in the phenomena of diffraction, which 

 present the effect, however, in such an unexpected and 

 minute form, that even Newton was misled, and turned 

 from the correct hypothesis of undulations which he had 

 partially entertained. 



In the third class of cases analogy fails us altogether, 

 and we expect that to exist which really does not exist. 

 Thus we fail to discover the phenomena of polarisation in 

 sound travelling through the atmosphere, since air is not 

 capable of any appreciable transverse undulations. These 

 failures of analogy are of peculiar interest, because they 

 make the mind aware of its superior powers. There have 

 been many philosophers who said that we can conceive 

 j nothing' in the intellect which we have not previously 

 : received through the senses. This is true in the sense 

 that we cannot image them to the mind in the concrete 



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