94 MEMOIR OF ARISTOTLE. 



and moist. Any of these 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, 



