"If they can understand reason, then why are you hitting them? And if they can't understand reason, then why are you hitting them?"
I agree. You shouldn't hit a child too young to understand why you're hitting them, and once they're old enough to understand, why the fuck would you not teach them instead of hitting them? Violence is never an appropriate way to "teach" anyone, especially a child, especially your own child. Every study on the subject shows it doesn't even work, and causes harm in the short and long term. Many countries have banned physical discipline. It can be done. And (surprise!) people still are able to raise kids fine. There's literally no reason to hit your child.