The problem of the lack of clarity in Zermelo's account was addressed by Weyl in 1910 (Weyl 1910; see especially p. 113) and then again by Skolem in 1922 (Skolem 1923, p. 139 of the reprint). What Weyl and Skolem both proposed, in effect, is that the question of what ‘definite properties’ are can be solved by taking these to be the properties expressed by 1-place predicate formulas in what we now call first-order logic. In effect, we thus have a recursive definition which makes the definite properties completely transparent by giving each time the precise route from ε, = to the definite property in question. This does not deal with all aspects of Poincaré's worry, but it does make it quite clear what definite properties are, and it does also accord with Zermelo's view that the relations =, ε are at root the only ones used.[32]