Full Idea
The equality of opposite reasonings is the cause of aporia; for it is when we reason on both [sides of a question] and it appears to us that everything can come about either way, that we are in a state of aporia about which of the two ways to take up.
Gist of Idea
Puzzles arise when reasoning seems equal on both sides
Source
Aristotle (Topics [c.331 BCE], 145b17), quoted by Vassilis Politis - Aristotle and the Metaphysics 3.1
Book Reference
Politis,Vasilis: 'Aristotle and the Metaphysics' [Routledge 2004], p.69
A Reaction
Other philosophers give up on the subject in this situation, but I love Aristotle because he takes this to be the place where philosophy begins.