From a letter of Jerome (84 in the PL edition):
Nec bonis adversariorum (si honestum quid habuerint) detrahendum est, nec amicorum laudanda sunt vitia; et unumquodque non personarum, sed rerum pondere judicandum est.
And we must not detract from the good qualities of our adversaries (if they have anything admirable to offer), nor praise the vices of our friends; and each individual thing must be judged on the weight not of the person who says it, but the things being said.
Apart from being a good example of untranslatable Latin (notice it took 48 English words to render what Latin said in 23—talk about economy!), this is an excellent lesson in civility. It also inspires a good rule for fruitful rational discourse: whenever someone says something we disagree with, we cannot really seek the truth with him unless we first try to understand what is good about what he said, presume good motives in what he said, and avoid alienation—the feeling of being other—that is, try to stand side by side, not opposite.
In a word, we ought to try to find at least one true thing that the other has said. Most of the time we are not being totally vicious, and so we are saying at least one true thing. Our starting point has to be finding out what that is in each other. Then we can seek truth together, because we have begun by actually doing so.
Thanks, dear reader, should you ever find me, for being my gadfly.