Full Idea
No one holds something as an opinion when he thinks that it is impossible for it to be otherwise - for then he thinks he understands it.
Gist of Idea
No one has mere belief about something if they think it HAS to be true
Source
Aristotle (Posterior Analytics [c.327 BCE], 89a07)
Book Reference
Aristotle: 'Posterior Analytics (2nd ed)', ed/tr. Barnes,Jonathan [OUP 1993], p.45