Social class is a recurring idea in the book. Philip not only has to contend with Rosemary when it comes to his love for Eleanor; he must also face the class divide, and to his credit, he does not flinch from it one inch.
The United States and New York City in particular have always been known as a place where dreams are made, where anything is possible, no matter what your beginnings. But in 1917, just as in present times, a class structure still thrives.