“…Forti and Hinnion 1989, Hinnion 1994, Hinnion and Libert 20032008) that show the consistency of various forms of Extensionality, but with highly restricted Abstraction: it is restricted to formulas in which neither negation, nor a conditional, nor even the abstraction operator itself occur. Because of these restrictions, these theories aren't naïve in our sense.…”