Lord Warburton approaches, and Isabel can tell at once that he has come to declare his feelings for her. She is deeply confused by this, as she has always considered men solely for their moral qualities and never for their positions of power and influence; she has never met a man as aristocratic as Warburton and worries that if she decides to reject him, she will be turning away a great social opportunity. Warburton asks her to stroll with him, and he quickly declares that he has fallen in love with her; he asks her to marry him. Isabel is deeply moved by his obvious sincerity, but she asks for some time before she gives him an answer. She feels that they do not know each other well and tells Warburton that she has not decided whether or not she ever wants to marry. Warburton leaves, and Isabel swiftly realizes that she does not want to marry him. She hopes she will be able to convince him that the marriage would be a bad idea, because she does not want to hurt him. She worries again that she is giving up a great opportunity by refusing the proposal and tells herself that she will have to do great things with her life to justify her decision. As she goes back into the house, Isabel feels strangely frightened of her own mind.
Isabel talks to Mr. Touchett about her decision; surprisingly, he has known of Warburton's intentions for three days, since he received a long letter from the lord asking his permission to marry Isabel. Isabel thinks about her other suitor, Caspar Goodwood; Goodwood is the son of a Boston cotton mill owner, who invented a device to improve the operation of the mill. He is a powerful person and a forceful manager of the mill, but he is possessive about Isabel and makes her feel confined. Isabel decides not to respond to Goodwood's letter; instead, she writes to Warburton, rejecting his proposal.
Henrietta tells Ralph that she is worried about the ways in which Isabel has changed since she came to Europe; Henrietta hopes that Isabel will marry Caspar Goodwood to prove her commitment to her old American attitudes. Complying with Henrietta's wishes, Ralph invites Goodwood to Gardencourt, but Goodwood declines the invitation. Henrietta decides to take Isabel on an excursion to London; when Ralph hears about this plan, he offers to go with them, implying that it would not be suitable for two women to take a trip to London unaccompanied by a man.
Lord Warburton comes to visit a few days later, accompanied by the elder Miss Molyneux. Isabel is again struck with the quiet contentment and simplicity with which Miss Molyneux goes about her life. After dinner, Isabel walks with Lord Warburton in the picture gallery. He is desperate to learn why she has rejected his proposal, and she replies that she cannot tell him until she can back up her thinking with evidence. He presses her, and eventually she tells him that she feels that to marry him would be to turn away from life and that she is committed to facing life directly. Henrietta, Ralph, and Miss Molyneux enter the gallery; Henrietta is badgering Miss Molyneux for information about the life of an aristocrat, just as she badgered Lord Warburton all through dinner. Henrietta wrangles an invitation to Lockleigh from Warburton.
That night, Mrs. Touchett comes to Isabel's room and asks her why she did not tell her about Warburton's proposal. Isabel replies that Mr. Touchett is better acquainted with Warburton. Mrs. Touchett says that she knows Isabel better, but Isabel says lightly that she is not sure. Mrs. Touchett later tells Isabel that she would have liked Isabel to marry Warburton, but when Isabel declares that she does not love him, Mrs. Touchett agrees that she did the right thing.
Isabel, Henrietta, and Ralph leave for a long trip to London, where Ralph finds Isabel more appealing than ever; she is bright and inquisitive and fascinated by everything she sees. One day, the three young people dine at the Touchetts' London home with Mr. Bantling, a friend of Ralph's. Mr. Bantling is taken with Henrietta and promises to obtain an invitation for her to visit the home of his sister, the Lady Pensil. Henrietta leaves to meet two of her friends from America, and Mr. Bantling decides to escort her.
Alone in the garden, Ralph and Isabel talk about Lord Warburton, whom Ralph praises highly. He understands that Isabel has rejected Lord Warburton out of a desire to remain free and independent. He says that he will be fascinated to watch Isabel's life unfold, because it is so surprising that a young woman would have thought her life more interesting without Lord Warburton in it. Isabel says that she merely wants to observe life. She takes her leave; Ralph wants to escort her back to the hotel room she shares with Henrietta, but Isabel says that he is clearly too exhausted. As Ralph helps her into her carriage, he thinks that his life is often troubled by people forgetting that he is an invalid but that it is much worse when people remember it.
Twelve chapters into the novel, Isabel has already faced two marriage proposals, one from the quintessentially American Caspar Goodwood, and one from the quintessentially English Lord Warburton. As Henrietta points out to Ralph, Isabel's romantic crisis with Warburton signifies the extent to which her American ideas and values have been affected by her time in Europe.
For all her life, Isabel has thought of men not as social opportunities, but as moral creatures, whom she admired or disliked based strictly on their personal qualities. With Lord Warburton, however, after having been attracted by the lives of the Misses Molyneux, Isabel suddenly has a powerful awareness that to marry into the English nobility would represent an extraordinary social opportunity. Of course Isabel eventually rejects this thought and then rejects Lord Warburton, but she is surprised that she has it in the first place—clearly, as Henrietta says, Europe has begun to change her.
Henrietta's solution to this problem is that Isabel should marry Caspar Goodwood, the symbol of the American character. For all her commitment to independence, Isabel cannot seem to escape the tendency of those around her to conceive of her destiny in terms of marriage and romance: who Isabel is, even to the fiercely democratic Henrietta, is to some extent a question to be answered based on whom she chooses to marry. This trend is further exemplified by Mr. Touchett, who is glad that Isabel did not marry Warburton, and Mrs. Touchett, who wishes that she had decided to marry him.
Only Ralph, the moral center of Portrait to a Lady, can see beyond this pitfall and conceive of a truly independent life for Isabel, where she will be able to think, act, and be exactly as she pleases. Ralph conveys all this to Isabel in their conversation at the end of Chapter 15, by which time another development should be obvious to the reader: though he denies it even to himself, Ralph is clearly in love with Isabel.