In the previous act, Joe Keller’s guilt in shipping faulty airplane
Mother: It’s too late, Joe. He knows.
Keller: [apprehensively] How does he know?
Mother: He guessed a long time ago.
Keller: I don’t like that.
Keller: [desperately, lost] For you, Kate, for both of your, that’s all I ever lived for …
Mother: I know, darling, I know …
For full extended scene, please refer to clips or the script edition cited here: Miller, Arthur. All My Sons. Samuel French, 1947.