G.K. Chesterton, St. Thomas Aquinas

At the top of his fury, Thomas Aquinas understands, what so many defenders of orthodoxy will not understand. It is no good to tell an atheist that he is an atheist; or to charge a denier of immortality with the infamy of denying it; or to imagine that one can force an opponent to admit he is wrong, by proving that he is wrong on somebody else's principles, but not on his own. After the great example of St. Thomas, the principle stands, or ought always to have stood established; that we must either not argue with a man at all, or we must argue on his grounds and not ours.

[Chapter III, "The Aristotelian Revolution"]