Anhedonia and It (pp. 695–696)
It is also lonely on a level that cannot be conveyed. There is no way Kate Gompert could ever even begin to make someone else understand what clinical depression feels like, not even another person who is herself clinically depressed, because a person in such a state is incapable of empathy with any other living thing. This anhedonic Inability To Identify is also an integral part of It. […] a clinically depressed person cannot even perceive any other person or thing as independent of the universal pain that is digesting her cell by cell. Everything is part of the problem, and there is no solution. It is a hell for one.
Context
It's loneliness clause — the anhedonic Unable To Identify folded inside the predator, so the two species turn out nested rather than parallel. "A hell for one" is the theme's shortest complete statement.