Full Idea
There is no better way of uncovering the truth and keeping falsity in check than by arguing with people who disagree with you.
Gist of Idea
Arguing with opponents uncovers truths, and restrains falsehoods
Source
Thomas Aquinas (On the spiritual perfection of life [1268], 26), quoted by Kretzmann/Stump - Aquinas, Thomas 05
Book Reference
'Shorter Routledge Encyclopaedia of Philosophy', ed/tr. Craig,Edward [Routledge 2005], p.35
A Reaction
Not the sort of attitude you associate with medieval scholastics, who are presumed to be dogmatists. How many modern philosophers actually have the courage to follow this advice?