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 lie 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- 
tion 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 
and moist ; earth, of cold and dry ; water, of cold 
