Edgar opens Act IV alone, talking himself into equanimity. He reasons that being at the bottom has one advantage: you cannot fall further. "The worst returns to laughter," he tells himself. "Welcome then, thou unsubstantial air."
Then he sees his father.
Gloucester is being led by an old tenant: eyeless, shuffling, wholly dependent on someone else's arm. Edgar's composure breaks instantly: "World, world, O world!" He steps back into his Poor Tom disguise before Gloucester can identify him.
Gloucester sends the old man away. The man protests: "You cannot see your way." Gloucester answers: "I have no way, and therefore want no eyes; I stumbled when I saw." It is the scene's pivot, and possibly the play's. The man who spent his life seeing what he wanted to see, who could not read either of his sons correctly, now understands what his sight was actually worth.
He says aloud what he cannot know Edgar is hearing: "O dear son Edgar, the food of thy abused father's wrath! Might I but live to see thee in my touch, I'd say I had eyes again."
Edgar, watching from inside his disguise, speaks in asides. "O gods! Who is't can say 'I am at the worst'? I am worse than e'er I was." And then: "The worst is not, so long as we can say 'This is the worst.'" He had told himself a moment ago that he had reached the bottom. He had not.
Gloucester speaks the line that defines the play's bleakest register: "As flies to wanton boys are we to the gods; they kill us for their sport."
He asks Poor Tom if he knows the way to Dover. There is a cliff there, he says: "whose high and bending head / Looks fearfully in the confined deep." Gloucester wants to be led to its edge. "From that place I shall no leading need."
He is not hiding his intent. Edgar understands exactly what he is agreeing to lead his father toward. He agrees anyway; knowing, we sense, that he will have to find another way to intervene before they reach the edge.
"Give me thy arm," Edgar says. "Poor Tom shall lead thee."
