Full Idea
Assume the rational soul has two parts, one to contemplate things with invariable first principles, one to contemplate variable things.
Clarification
'Soul' is the Greek word 'psuché', which covers mind and consciousness and life
Gist of Idea
Assume our reason is in two parts, one for permanent first principles, and one for variable things
Source
Aristotle (Nicomachean Ethics [c.334 BCE], 1139a06)
Book Reference
Aristotle: 'Ethics (Nicomachean)', ed/tr. ThomsonJ A K/TredennickH [Penguin 1976], p.204
A Reaction
'Assume' is interesting. He presumably isn't asserting this division as a fact. So his methodology is make assumptions - probably as aids to clear thinking.