Full Idea
Democritus says the soul is a sort of fire. For the shapes and atoms are unlimitied and those that are spherical he says are fire and soul - which are like the motes in the air when sunbeams come through the window.
Gist of Idea
Democritus says spherical atoms are fire, and constitute the soul [psuche]
Source
report of Democritus (fragments/reports [c.431 BCE], DK 67-68) by Aristotle - De Anima 403b31
A Reaction
It's hard to see why the spherical atoms should be fire. Maybe because they move together quickly and easily. …At 404a5 Aristotle agrees with me!
Book Reference
Aristotle: 'De Anima (on the psuche)', ed/tr. Reeve, C.D.C. [Hackett 2017], p.5