With no explanation, label text_A→text_B with either "DON'T KNOW", "NO" or "YES".
text_A: A real number "a" is first-order definable in the language of set theory, without parameters, if there is a formula "φ" in the language of set theory, with one free variable, such that "a" is the unique real number such that "φ"("a") holds (see). Using the language of set theory, it is impossible to express this notion as a formula.
text_B: Does the passage make it sound like a mathematical discovery that came after set theory, or that is more complex than set theory, is necessary to express this notion?
YES.