There's a soberness in Mark's expression as he takes this in, but when Arthur poses that question to him directly, his eyes slide away to the floor, as if bearing some weight too heavy to keep his gaze aloft to hold the King's. The inquiry seems to give him a great amount of pause, and it's obvious that he's struggling to respond, even if it's meant to be rhetorical. Finally, he takes a different tack entirely.

"You said your mistakes have taught you a lot. What would you consider the most important lesson?"