 The Russellian approach did not encounter among mathematicians a success comparable to that of set theory. Nevertheless, from an ontological point of view, it appears to be more economical in its relying on properties, to the extent that the latter are needed any way for all sorts of explanatory jobs, reviewed above, which sets, qua extensional entities, can hardly perform. As we have seen, type theory is problematic. However, type-free property theories can come to the rescue by replacing typed properties with untyped ones in the foundations of mathematics. And in fact the advocates of such theories have often proposed to recover the logicist program, in particular by identifying natural numbers with untyped properties of properties (Bealer 1982; Cocchiarella 1986b; Orilia 2000b). (See also entry on  logicism and neologicism).