Charles Mauron was Forster's French translator and he was an important literary critic of the first half of the twentieth century. In her book Formalism and the Freudian Aesthetic, Linda Hutcheon argues that Mauron's psychological criticism influenced Forster's division of characters into flat and round ones in Aspects of the Novel. In The Nature of Beauty in Art and Literature, Mauron suggests a division in character types and implies that the psychological realities embodied by the characters provoke an aesthetic response in the readers.

You can read the passage from Hutcheon's work.