Reading the last sentence of Ellison’s Invisible Man, I stumbled a bit on the phrase “but that:”
Being invisible and without substance, a disembodied voice, as it were, what else could I do? What else but try to tell you what was really happening when your eyes were looking through? And it is this which frightens me: Who knows but that, on lower frequencies, I speak for you?
A compelling sentence, but I’m not so sure of the meaning of “but that.” My dictionary says another phrase, “to the contrary,” can be substituted for it. But if I were to guess now, Ellison’s narrator seems to be saying, with some apprehension: “Who knows for certain that, on lower frequencies, I don’t speak for you?”