Let us remember that this play is above all else about the way in which society invades personal lives, and the tragic consequences of such a fact. Mrs. Alving's famous speech in Act II of the play establishes the key metaphor that gives the play its title:
I am half inclined to think we are all ghosts, Mr. Manders. It is not only what we have inherited from our fathers and mothers that exists again in us, but all sorts of old dead ideas and all kinds of old dead beliefs and things of that kind. They are not actually alive in us; but there they are dormant, all the same, and we can never be rid of them. Whenever I take up a newspaper and read it, I fancy I see ghosts creeping between the lines. There must be ghosts all over the world. They must be as countless as the grains of the sands, it seems to me. And we are so miserably afraid of the light, all of us.
Note the way that the "ghosts" of public opinion and duty are shown to be so powerful and to exert control over the living. These "ghosts" have the power to haunt, ruin and dominate generations of lives, as Mrs Alving's life and the way she is held captive by maintaining the fiction of her husband's life shows. Of course, this is something that comes to ruin her children's life as well as her own life. As people, we are not only tormented by our individual "inheritances" from people, but also by the various supersitions that are present in every community. In this play, the duty towards members of your family is such a ghost that is shown to be able to ruin lives. Mrs. Alving's morose speech points towards the tremendous power such "ghosts" exert and how hard it is to escape their influence.