"This is what led Democritus to say that soul is a sort of fire or hot substance; his 'forms' or atoms are infinite in number; those which are spherical he calls fire and soul, and compares them to the motes in the air which we see in shafts of light coming through windows."
— Aristotle (384-322 B.C.)
(403b30-404a15, p. 160)
Reading in Aristotle, Introduction to Aristotle, trans. R. McKeon. (Chicago and London: The University of Chicago Press, 1973).