I 



What are Animals and Plants f 371 



Thus, as regards the science of astronomy, who does not 

 now see that our conceptions of the motions of the heavenly- 

 bodies have been greatly facilitated by the discovery of the 

 law of gravitation ? and who does not perceive in the verifica- 

 tion of scientific prophecy, by the discovery of the planet 

 Neptune, a signal triumph of modern astronomical science ? 



Nevertheless, the fulfilment of predictions alone will not 

 always suffice to prove the absolute truth of the views upon 

 which they are supposed to be based, or else the prediction of 

 eclipses by astronomers who followed the Ptolemaic system 

 would have proved the truth of that erroneous theory. 



Bearing in mind, however, the aim and end of physical 

 science, let us next glance at the only means which it is in 

 the power of scientific men to use. These means are the 

 employment of present sense-impressions, together with the 

 reproduction in the investigation of groups of past sense- 

 impressions. 



All our knowledge is called forth by the play of surround- 

 ing nature upon our sense-organs ; nor can we imagine any- 

 thmg which we have not previously had sensuous experience 

 of — at least in its elements or component parts. 



Again, there is a quality of distinctness and vividness 

 in our sense-impressions. How vague, for example, is our 

 imagination of a perfume, compared with our imagination of 

 a visible triangular figure, or of a cube, or of a ball, held in the 

 hand? 



It is especially what is visible and tangible that comes 

 home most readily to the imagination ; vague internal sensa- 

 tions are always described by us in terms of sight or touch. 

 We speak of a ' gnawing ' pain, a ' sharp ' pain, like a knife, a 

 'rough' taste, and even Si'bright' intellect, and a ' hard ' heart.^ 



Now, the 'explanation' of any phenomenon may be its 

 reference to the causes which produce it ; but its ' explana- 



1 See ante, p. 282. 



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