Psmith expresses a humorous skepticism about the existence of Clapham Common, a well-known location in London. He mentions that while people often talk about it, he questions whether it has ever been definitively proven to exist. This suggests a playful approach to common knowledge, highlighting how things can be accepted without question in society.

By raising this point, Psmith introduces a whimsical notion that challenges the assumptions of everyday life. His quizzical attitude reflects a larger theme in P.G. Wodehouse's writing, where characters often engage in witty banter that reveals deeper insights about their world and the absurdities within it.