Essayist: Common sense, which is always progressing, is nothing but a collection of theories that have been tested over time and found useful. When alternative theories that prove even more useful are developed, they gradually take the place of theories already embodied in common sense. This causes common sense to progress, but, because it absorbs new theories slowly, it always contains some obsolete theories.
At least some theories that have been tested over time and found useful are less useful than some other theories that have not been fully absorbed into the body of common sense must be true if all of the essayist's statements are true.