From John Piper's Think:
Carnell on the Moral Dimension of Rationality
In 1957 Edward John Carnell published a powerful book called Christian Commitment: An Apologetic. Apart from the Scriptures it was probably this book that opened my eyes most to the moral dimension of rationality. In other words, Carnell made clear that there is a profound sense in which being irrational is immoral. He went beyond Descartes’ “I think, therefore, I am” and argued, “I think, therefore I am morally obliged to admit the reality of my own existence.”1 Human existence and logical inference are intrinsically moral. He illustrates:
When Aristotle tried to refute the skeptics, however, he encountered the frustrating fact that the skeptics used the law of contradiction to deny the law of contradiction. . . . After exhausting all his dialectical powers, Aristotle had to bow to the truth that only men of character can apprehend rational ultimates. . . . Aristotle, like Kant, illuminates the fact that the rational life cannot get on with it unless the moral life is firm.
The Games Professors Play at School
The immoral dimension of relativism is most obvious when relativists live their lives. They simply do not live them as though relativism were true. Professors may play the academic game of relativism in their classes, but when they go home they get upset when their wives don’t understand what they say. Why do they get upset? Because they know that there is an objective meaning that can be transmitted between two human beings, and we have moral obliga tions to grasp what is meant.
No husband ever said, “Since all truth and language are relative, it does not matter how you interpret my invitation to sleep together.” Whether we write love letters, or rental agreements, or instructions to our children, or directions for a friend, or contracts, or sermons, or obituaries, we believe objective meaning exists in what we write, and we expect people to try to understand. And we hold them accountable (and often get upset) if they don’t.
Nobody is a relativist when his case is being tried in court and his objective innocence hangs on objective evidence. The whole system of relativism is a morally corrupting impulse. It brings with it duplicity and hypocrisy. It is a great bluff. And what is needed in our day is for many candid children to rise up as in the fairy tale and say, “The king has no clothes on.”