i«4 A N T I E N T M E T A P H Y S I C S. Book II. 



of thefe two*.- Ariflotle fuppofeSj that the motion in the ftraight 

 line is either /o the center, as heavy bodies are moved; ot from 

 the center, as light bodies are moved. But, had he obferved thofe 

 motions of the fmall particles of matter which chcmiftry exhibits, 

 he would have been convinced, that bodies have a tendency to move, 

 both to a.nd J'rom one another, in every diredion ; and Empedocles, 

 wlio made friendlhip and enmity two of the principles of nature, 

 would have feen his hypothefis wonderfully verified in the attradions, 

 as they are called, and repulfions of chemical bodies. Thefe are the 

 fads w^hich mud be the bafis of any fyftem of philofophy upon this 

 fubjed ; but they are no more than fads ; and philofophy is the know- 

 ledge of cau/cs. We muft, therefore, inquire what is the cau/e of all 

 thofe various motions of body. 



And one cauje is obvious to common obfervation, namely, matter 

 ading upon matter. For, that one body moves another by triifi07idin<\ 

 pulfwn, or, as it may be exprefled in one word, by impuJje^ is a fad 

 of daily experience. And, not only are all the works of art perform- 

 ed in this way, but a great part of the bufinefs of nature is carried on 

 by motion communicated from one body to another ; for, that there 

 is a great deal of machinery in nature, is known, not only to men of 

 fcience, but to every common obferver. We may therefore lay it 

 down, as a law of nature, that body moves body by impidfe. 



But, for this purpofe, it is neceflary, in \S\^ firjl place, that the body 

 moving (liould be 7noved itfelf, and by fome other cau/e than itfelf ; as 

 I think I have dcmonflrated, that nothing can move itfelf. This 



caufcy 



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