"Who sanitizes the sanitizer?"
I saw this cartoon in today's New Yorker.
The question sets off an infinite regress.
This is the problem Plato and Aristotle dealt with; viz., how to "spike" the infinite regress. Is it Plato's Forms? Or Aristotle's "Unmoved Mover."
The question (its form; its implication) - even today - fuels metaethical discussions.