 Since the systematization in the first half of last century, which gave rise to paradox-free axiomatizations of set theory such as ZFC, sets are typically taken for granted in the foundations of mathematics and it is well known that they can do all the works that numbers can do. This has led to the proposal of identifying numbers with sets. Russell’s type theory was an alternative that rather relied on properties (viewed as propositional functions), in backing up a logicist reduction of mathematics to logic. In essence, the idea was that properties can do all the work that sets are supposed to do, thus making the latter dispensable. Hence, Russell spoke of his approach as a “no-classes” theory of classes (see Landini 2011: 115, and §2.4 of the entry on  Russell’s logical atomism;  cf. Bealer 1982: 111–119, and Jubien 1989 for followers of this line). Following this line, numbers are then seen as properties rather than sets.