Inner-Model Reflection Principles

We introduce and consider the inner-model reflection principle, which asserts that whenever a statement φ(a) in the first-order language of set theory is true in the set-theoretic universe V, then it is also true in a proper inner model W ⊆ V. A stronger principle, the ground-model reflection principle, asserts that any such φ(a) true in V is also true in some non-trivial ground model of the universe with respect to set forcing. These principles each express a form of width reflection in contrast to the usual height reflection of the Lévy-Montague reflection theorem. They are each equiconsistent with ZFC and indeed П₂-conservative over ZFC, being forceable by class forcing while preserving any desired rank-initial segment of the universe. Furthermore, the inner-model reflection principle is a consequence of the existence of sufficient large cardinals, and lightface formulations of the reflection principles follow from the maximality principle and from the innermodel hypothesis IMH. We also consider some questions concerning the expressibility of the principles.