Which sentence in this excerpt from Leo Tolstoy's The Death of Ivan Ilyich shows that Ivan Ilyich finally understands the cause of his suffering?

Suddenly some force struck him in the chest and side, making it still harder to breathe, and he fell through the hole and there at the bottom was a light. What had happened to him was like the sensation one sometimes experiences in a railway carriage when one thinks one is going backwards while one is really going forwards and suddenly becomes aware of the real direction.

"Yes, it was not the right thing," he said to himself, "but that's no matter. It can be done. But what is the right thing? he asked himself, and suddenly grew quiet.

This occurred at the end of the third day, two hours before his death. Just then his schoolboy son had crept softly in and gone up to the bedside. The dying man was still screaming desperately and waving his arms. His hand fell on the boy's head, and the boy caught it, pressed it to his lips, and began to cry.

At that very moment Ivan Ilyich fell through and caught sight of the light, and it was revealed to him that though his life had not been what it should have been, this could still be rectified. He asked himself, "What is the right thing?" and grew still, listening. Then he felt that someone was kissing his hand. He opened his eyes, looked at his son, and felt sorry for him. His wife camp up to him and he glanced at her. She was gazing at him open-mouthed, with undried tears on her nose and cheek and a despairing look on her face. He felt sorry for her too.

"Yes, I am making them wretched," he thought. "They are sorry, but it will be better for them when I die." He wished to say this but had not the strength to utter it. "Besides, why speak? I must act," he thought. with a look at his wife he indicated his son and said: "Take him away...sorry for him. ..sorry for you too. ..." He tried to add, "Forgive me," but said "Forego" and waved his hand, knowing that He whose understanding mattered would understand.