That the structure V = UaVa be the universe of all sets, that the set theoretical axioms be true assertions about V or that a question like the Continuum Hypothesis be still open (since it is undecided whether it holds or fails in V) are common assertions in set theory. How one is to understand them, tough, is not an obvious matter. The aim of this book is just to interpret the façon de parler that V is the universe of all sets in a way that is faithful to what is actually done in set theory.