We propose a "natural" axiomatic theory of the Foundations of Mathematics (Theory Q) where, in addition to the membership relation (between elements and classes), pairs, sets, natural numbers, n-tuples and operations are also introduced as primitives by means of suitable ground classes. Moreover, the theory Q allows an easy introduction of other mathematical and logical entities. The theory Q is finitely axiomatized in § 2, using a first-order language with a binary relation $\in$ (membership) and five constants (ground classes), and it is shown to be equiconsistent with Gödel-Bernays class theory; in fact, in § 3; both these theories are mutually interpreted inside each other.