"Only one conciousness per mind." (NOT brain.) Possibly tautological; "Only one mind per mind." Possibly wrong; IIRC one theory of "voices" in schizophrenia is intrusion of normally subconcious sub-personalities into the main personality i.e. you're actually hearing someone else's thoughts!
A frequently quoted book, "Man on his Nature", Sherrington looks more valuable than this essay...because properly scientific.