58 THE AIM AND ACHIEVEMENTS OF SCIENTIFIC METHOD. 



suggestion. Keppler has been reading the newly-published 

 treatise De Magneto, by Gilbert of Colchester, the Father of 

 Experimental Science, and has fastened upon the fruitful 

 analogy between magnetic and stellar phenomena. The first 

 result is a " true doctrine of gravity " which points directly to 

 the completer doctrine of Newton. Kepeating the argument 

 given above in connexion with the anima mundi Keppler 

 asserts the impossibility " ut forma lapidis movendo corpus suum 

 quaerat punctum mathematicurn aut mundi medium."* On the 

 contrary, "gra vitas est affectio corporea mutua inter cognata 

 corpora ad unitionem sen conjunctionem (quo rerum ordine est 

 facultas maynetica) ut multo magis Terra trahat lapidem quam 

 lapis petit Terram."f There are, it is true, difficulties in the 

 application of the analogy. The investigations of a Galileo were 

 necessary before a Newton could see that the moon is actually 

 and always falling towards the earth. For Keppler the 

 difficulty is to account for their remaining apart : " Si Luna et 

 Terra non retinerentur m animali aut alia aliqua aequipollenti 

 . . . Terra ascenderet ad Lunam . . . Luna descenderet 

 ad Terrain . . . ibique jungerentur."| 



The words italicised in this passage illustrate at once 

 Keppler's willingness to retain the conception of the anima 

 mundi and his growing preference for a facultas corporea to 

 a facultas animalis if the former can make the facts intelligible. 

 We may leave the consideration of the development of his 

 ideas at the point where he reaches a " secondary construction " 

 of the facts of the stellar observations suggested altogether by 

 such material analogies. In this conception the planetary 

 movements are ascribed to a two-fold "virtue" one of the 

 planet and one of the sun. That "of the planet is compared 

 with the work of oars in rowing, that of the sun to the stream 

 of the river. And so we reach the all-important conclusion, 

 in which the soundness of this conception is based upon the 



* Introd., p. 150. t Lvtrod., p. 151. % Ibid. 



