What is the theme in The Ghost Sonata by August Strindberg?

Illusion vs. Reality is a theme. Strindberg liked to view himself as a continual seeker of truth, as an artist who could present the sin, suffering and degradation of the world on the stage and unmask all of the world's liars, hypocrites and criminals. Several of his plays attempted to reveal what he felt were the hidden secrets of his society its institutions and individuals. In a letter to his friend Emil Schering dating March 27, 1907, Strindberg wrote of The Ghost Sonata, "It is horrible, like life, when the veil falls from our eyes and we see things as they are. Secrets like these are to be found in every home. People are too proud to admit it; most of them boast of their imagined luck, and hide their misery."