Every set has a natural number assigned to it, called its level. The simplest sets, those of level 0,
are called individuals—they do not have elements. A collection of individuals is a set of level 1; a
collection of sets of level 1 is a set of level 2; and so on. In the theory of types the expression a ∈ B
is only meaningful if, for some number n, a is a set of level n and B is a set of level n + 1. It follows
that the statement x ∈ x has no meaning in the theory of types, and as a result, Russell’s paradox
vanishes for the simple reason that it cannot even be formulated.