The basic problem is that it is not explained by Zermelo what the precise route is from the fundamental relations ε and = to a given ‘definite property’; it is this which gives rise to a general doubt that the Separation Axiom is not, in fact, a safe replacement for the comprehension principle (see Fraenkel 1927: 104). This plays into the hands of those, who, like Poincaré, consider adoption of the Separation Axiom as insufficiently radical in the search for a solution to the paradoxes. Poincaré writes: