According to chapter 6, what does David say he learned from his stay in the house?

The next morning, David becomes more interested in Sophie's photo albums, and in one picture in particular - that of a woman that Sophie identifies as Edith Hjorth Fengel. David says "her eyes look as if - as if she'd known a great deal, and yet she's smiling." Sophie tells him how Edith and her husband had been politically rebellious, how they had been taken prisoner along with their one-year-old son—the boy David she said earlier she had never seen— how her husband and child had both been executed, and how she had escaped to Denmark with the help of a compassionate guard. Sophie goes on to comment that even after all her unhappiness, Edith is as happy "as you can be when you've gone through great unhappiness." She then tells David that sorrow does not last forever (see "Quotes," p. 155).