Abstract

In several areas of Mathematical Logic and Computer Science one would ideally like to use the set F orm(L) of all formulas of some first-order language L for some goal, but this cannot be done safely. In such a case it is necessary to select a subset of F orm(L) that can safely be used. Three main examples of this phenomenon are: • The main principle of naive set theory is the comprehension schema: ∃Z(∀x.x ∈ Z ⇔ A)