How does one define recursion in the context of wff in ZFC? Seems impossible, but simultaneously necessary. If we can't define recursion, we can't define addition, but no wff can be infinite, so no one formula can deal with the infinite natural numbers
I thought I could use Replacement, but you need to pick a particular instance of Replacement to work with all natural numbers, but thatbdoesnt work when the natural numbers are infinitr