Read the following excerpt from the novel The Portrait of a Lady by Henry James:
“Isabel Archer was the most strikingly beautiful girl I had ever seen. She had a certain vivacity—an unknowing sparkle in her eyes but a resolute expression that gave no room for compromise. Having traveled from America to Europe, she faced the world with a blend of independence and naiveté that often left those around her either enchanted or exasperated. Yet, despite her allure, there was a persistent undercurrent of yearning that seemed to shadow her every move.”
Based on this passage, what is the central theme regarding Isabel Archer's character as presented by the author?