Full Idea
Alcmaeon says that the soul is immortal because it resembles immortal things and that this affection belongs to it because it is always in movement, like divine things, such the moon, the sun, the stars and the whole heaven.
Gist of Idea
Soul must be immortal, since it continually moves, like the heavens
Source
report of Alcmaeon (fragments/reports [c.490 BCE], DK 24) by Aristotle - De Anima 405a30
A Reaction
Hm. Fish and rivers seem to be continually moving too. Presumably we are like gods, but then Greek gods seem awfully like humans. I don't know the history of belief in immortality; an interesting topic.
Book Reference
Aristotle: 'De Anima (on the psuche)', ed/tr. Reeve, C.D.C. [Hackett 2017], p.8