Reading my ethics homework, and I can’t help but wonder why we’re allowed to “pick and choose” when it comes to philosophy.

Why is Aristotle’s above quote widely recognized (at least I hope so) as false, but we accept many of his other proclamations as true? 
& this question isn’t entirely specific to Aristotle. If all early philosophers reflect the assumptions of their times, wouldn’t that falsify many of their theories when applied to modern contexts?
I think this might be why I’ve never really been interested in philosophy. I’m sure the discoveries of early philosophers can important to students’ philosophical foundations, but I firmly find using these men as all-knowing rule books to be super problematic. 

/useless post/tl;dr