It is a powerful and compelling sense of isolation which is different from all other experiences of aloneness and isolation. In those other experiences you could, if you had chosen, have contacted other people. From your camp in the woods you could have gone back home. From your lonely college room you could have found a phone and called home. But from the prison of depression there is no path, no telephone that will connect you with others.
All paths peter out, all telephone lines are down.
You are surrounded by a wall which, though it is invisible, is impenetrable. Outside the wall those who know you realize that the wall is there. They reach out to you and, though their hands may touch your flesh, what they feel is the wall which resists all their love and entreaties. To them the wall is as palpable as it is invisible. Banging our head against a wall is frustrating, but what makes the wall around a depressed person doubly frustrating is that the person inside knows and the person outside senses that in some way the person inside wants to be there.