His "devil twin", as he called it.

"which never sleeps, but talks to me forever of such things as they only speak of in Hell. No imagination can conceive the dreadful temptations it sets before me. For some unforgiven wickedness of my forefathers, I am knit to this fiend — for a fiend it surely is. I beg and beseech you to crush it out of human semblance, even if I die for it."