Socrates responds by pointing out that Thrasymachos is confusing the art of tending sheep with the art of profiting from them. The art of medicine aims to improve health, whether or not doctors get paid for practicing it. The art of shepherding aims to ensure the well-being of sheep, whether or not the shepherd (or his employer) is also a businessman who knows how to extract a profit from them. Just so with the art of governance.
