What lines best indicate King Lear's insanity?

In Act 4, Scene 6, Shakespeare gives a glimpse of how Lear's mind is working.
Look, look, a mouse! Peace, peace; this piece of toasted cheese will do 't.
Lear has been living in the open fields and evidently has actually been reduced to eating mice and anything else he can find. He is like Poor Tom, who told him in Act 3, Scene 4:
But mice and rats, and such small deer, Have been Tom's food for seven long year.
What is ironic about Lear is that he doesn't know whether he is a derelict in a field or a king in a palace. He is hungry and wants to eat the mouse, but he plans to tempt it nearer with a piece of toasted cheese. Here he is planning to use a gourmet tidbit to attract a mouse so that he can eat it. This indicates that he is mentally in two places at once: his palace and the open field near Dover. He is so far gone that he doesn't know where he is, but he can't quite let go of the notion that he is still the king. This little snatch of dialogue would be funny if it were not so pathetic.