94 
MEMOIR OF ARISTOTLE. 
and moist. Any of those elements may pass into 
another by the privation of one of the combined 
principles ; for instance, water into air by the priva- 
tion of cold, and the consequent union of hot with 
the moist that remains. When the change is simply 
in the affections or attributes of some existing body, 
the process is that of alteration ; but when the change 
involves an entire transmutation of the original ma- 
terial, the process is that of generation and corrup- 
tion. Upon these complex principles did Aristotle 
account for all the phenomena, sensible and tangible, 
that take place in the material universe around us. 
The heavenly luminaries, as constituting a branch 
of physics, demanded his attention from their neces- 
sary connexion with the full development of his 
theory of motion, and in order to trace up that prin- 
ciple through its successive impulses from this lower 
world to the First Cause or Prime Mover. His 
whole astronomy is dependent on those speculative 
notions which he had adopted of lightness and heavi- 
ness as intrinsic and absolute properties of bodies, 
by which the exact position of each of the material 
elements was regulated in the mundane system. 
Fire he placed in the extreme point upwards ; earth 
lowest ; and in the intermediate space, air and wa- 
ter. On some points, his notions were tolerably cor- 
rect. He admits the spherical form of the earth, 
from the evidence of lunar eclipses, in which he had 
remarked that it always exhibited a curved outline 
and he inferred its magnitude to be not very great, 
