MEMOIR OF ARISTOTLE. 93 



or a self- working power, he was not called upon to 

 show how those changes which took place in the 

 material world might be satisfactorily accounted for. 

 It was no part of his philosophy to demonstrate that 

 any particular material, or combination of materials, 

 was employed in these processes of nature for ef- 

 fecting her productions and transmutations. All he 

 assumes is, that some material or other is used in 

 every instance of a physical object, to effect that con- 

 stitution of it in which its " form" consists. From 

 considering this question, he proceeds to examine 

 what principles reject and exclude one another in 

 the various changes of the material world ; these be- 

 ing the causes of the transition of one nature into 

 another : the presence of one involving the priva- 

 tien of all those forms of matter dependent on the 

 other. What these mutually excluding principles 

 are, he decides by a reference to the sense of touch ; 

 that being the proper evidence to us of the existence 

 of body, as may be inferred from its resistance to 

 that faculty. According to this theory, the contra- 

 rieties ascertained by touch, and which account for all 

 the different forms of matter, are hot and cold, dry and 

 moist ; the first two as active principles, the last two 

 as passive. These four principles admit only of four 

 combinations ; it being impossible that the contraries 

 of heat and cold, or moist and dry, can co-exist. 

 The effect of each combination is a different element ; 

 thus, fire is a coalition of hot and dry ; air, of hot 

 aaid moist ; earth, of cold and dry ; water, of cold 



