Truth is truth, and error, error, and that which is unlawful is unlawful, whether men think so or not. God has put an eternal difference between light and darkness, good and ill, which no creature's conceit can alter; and therefore no man's judgment is the measure of things further than it agrees to truth stamped upon things themselves by God. For this reason, because a wise man's judgment agrees to the truth of things, a wise man may in some sense be said to be the measure of things, and the judgment of one holy wise man to be preferred before a thousand others. Such men usually are immovable as the sun in its course, because they think, and speak and live by rule. A Joshua and his house will serve God (Josh. 24:15), whatsoever others do, and will run a course contrary to the world, because their judgments lead them a contrary way. Hence it is that Satan has a spite at the eye of the soul, the judgment, to put it out by ignorance and false reason, for he cannot rule in any until either he has taken away or perverted judgment. He is a prince of darkness, and rules in darkness of the understanding.
-- p. 84-85, The Bruised Reed by Richard Sibbes
This reminded me Tim Challies' book, The Discipline of Spiritual Discernment. Richard Sibbes couldn't have had the post-modernists who are plagued by relativism when he preached/penned this since he's from 17th century, but this little text sounds very timely for a culture like ours.