This means that, for any variable symbol formula_17 the formula formula_16 can be regarded as an axiom. Also, in this example, for this not to fall into vagueness and a never-ending series of "primitive notions", either a precise notion of what we mean by formula_16 (or, for that matter, "to be equal") has to be well established first, or a purely formal and syntactical usage of the symbol formula_20 has to be enforced, only regarding it as a string and only a string of symbols, and mathematical logic does indeed do that.