How we cite our quotes: (Book.Chapter.Paragraph)
Quote #7
'Of course they're totally different kinds of duty.' (10.3.11)
Strether tries to argue with Sarah by saying that there are different kinds of duty. In other words, Chad has duties to his family, but also to Madame de Vionnet because she is responsible for all of his improvement as a person. Of course, Strether doesn't realize at this point that Sarah doesn't approve of Chad's transformation at all. When it comes to duty, they're speaking two different languages.
Quote #8
'You can sacrifice mothers and sisters to her without a blush, and can make them cross the ocean on purpose to feel the more, and take from you the straighter, how you do it?' (10.3.14)
As Strether continues to talk with Sarah, it becomes pretty obvious that there's no argument he can possibly make that'll convince her of Chad's duty to Madame de Vionnet. Sarah regards Chad's desertion of his family as a total abomination. Period. No more discussion. She's a total brick wall when it comes to this issue.
Quote #9
'You owe her everything—very much more than she can ever owe you. You've in other words duties to her, of the most positive sort; and I don't see what other duties—as the other are presented to you—can be held to go before them.' (12.4.34)
In their final conversation, Strether tells Chad that he (Chad) owes everything to Madame de Vionnet. Chad still has a lingering sense of duty to return to his family in Woollett, but Strether tells him in very clear terms that his duty now lies with being a loving partner to Madame de Vionnet. This is big news! It's the exact opposite of what Strether believed at the beginning of the book.