John Stuart Mill on truth and debate

I just discovered this quote from one of Jordan Peterson’s videos:

He who knows only his own side of the case knows little of that. His reasons may be good, and no one may have been able to refute them. But if he is equally unable to refute the reasons on the opposite side, if the does not so much as know what they are, he has no ground for preferring either opinion.
John Stuart Mill, 1859

My motto has always been: “You can’t argue against something until you can argue for it.”