An unthinkable thought is not one that hasn't occurred to somebody, nor is it a thought that somebody considers to be wrong. An unthinkable thought threatens a person's entire existence and is therefore subversive and consequently can be thought of and has been thought of, but has been pushed out of the mind's currency and subsumed into its margins where it festers. Dark nights of the soul are lit by inconceivable ideas. Any story may draw its source from the power of an unthinkable thought.
