In his booklet Absolute Truth, Mark Ashton tells the following story about a professor who demonstrates that moral relativism is unlivable:

Roger Wengert, a philosophy professor at the University of Illinois, often begins his introductory ethics classes by asking how many of the students believe that truth is relative. A show of hands usually reveals that two-thirds to three-fourths of the class thinks in this manner. After discussing the syllabus, testing dates, papers and content of the course, Wengert informs the class that they will be graded according to height. When the smart-alecky tall kid loudly agrees with this system, the professor adds, "Short students get A's; tall students flunk."

Inevitably a student's hand is raised: "Your grading system is not fair." "I am the professor," retorts Wengert. "I can grade however I wish." The student insists, "But what you ought to do is grade us according to how well we learn the material. You should look at our papers and exams to see how well we have understood the content of the course and grade us on that." The class nods in affirmation (especially the tall students).

Professor Wengert then replies, "By using words like should and ought, you betray your alleged conviction that truth is relative. If you were a true relativist, you would realize that there is no external standard to which my grading should conform. If my truth and my ethic lead me to an alternate grading system that you deem inappropriate, c'est la vie! I will grade however I wish."