 More precisely, Peacocke holds that understanding of metaphysical modality, namely the concepts of possibility and necessity, consists in having implicit knowledge of what he calls Principles of Possibility, which include principles concerning the rules of concept use, as well as constitutive (or essentialist) principles. These principles govern understanding and evaluation of modal discourse and accordingly are implicitly known and deployed by subjects that correctly understand the modal concepts. Crucially, for Peacocke, these principles are necessary truths, and understanding and being able to deploy them is essential to modal thinking and evaluation. As he puts it,