Set theory is concerned with a “domain” 𝔅 of   individuals, which we shall call simply “objects” and   among which are the “sets”. If two symbols, a   and b, denote the same object, we write a = b,   otherwise a ≠ b. We say of an object a that it “exists” if it belongs to the domain 𝔅; likewise we say of a class 𝔎 of objects that “there exist objects of the class 𝔎” if 𝔅 contains at least one individual of this class. (1908b: 262)