Chapter 3 of the dogmatic constitution on the Church of Vatican Council I (Pastor aeternus) is the principal document of the Magisterium about the content and nature of the primatial power of the Roman Pontiff. Chapter 4 is a development and defining of one particular characteristic of this primatial power, namely the Pope's supreme teaching authority, i.e. when the Pope speaks ex cathedra he teaches the doctrine of the faith infallibly.