  Of course, this first argument leaves open the possibility that set theory is a special case—that, even though some branches of mathematics, like number theory and analysis, should be understood absolutely, set theory, like group theory and topology, should still be understood algebraically. Unfortunately, there are a number of obvious differences between the practice of set theory and that of more-clearly algebraic subjects like group theory. So, for instance, mathematicians tend to treat the axioms of set theory as being less fixed than those of group theory or topology. In set theory, mathematicians sometimes raise the question as to whether the ZFC axioms are correct—i.e., they talk as though there is an intuitive notion of set against which the ZFC axioms might be checked and found wanting. In group theory and topology, by contrast, it simply makes no sense to talk about “intuitive notions” which could diverge from the notion specified by the relevant  axioms.[42]  In a similar vein, set theorists sometimes debate whether we should add new axioms to the standard axioms of set theory—e.g., large cardinal axioms, or axioms like V=L, or even just axioms like Con(ZFC). In contrast, no one would dream of making additions to the axioms of group theory or topology. In this sense, then, an algebraic approach to set theory is revisionary of set-theoretic practice in a way that an algebraic approach to group theory is not.