The type of question Moore refers to in this argument is an identity question, "Is it true that X is "Y"?" Such a question is an "open question" if a conceptually competent speaker can question this; otherwise it is "closed." For example, "I know he is a vegan, but does he eat meat?" would be a closed question. However, "I know that it is pleasurable, but is it good?" is an open question; the answer cannot be derived from the meaning of the terms alone.