 The PSR is, in fact, a family of principles which are generated by various restrictions of (2), and by ascriptions of different degrees of modal strength to (2). To begin with, variants of the PSR may differ according to how they restrict the kinds of things that require a reason (the explananda). Thus, one might restrict the PSR to only actual entities, or include possibilia as well. Alternatively, one might formulate the PSR as requiring a sufficient reason for every (true) proposition or as pertaining to entities and their properties. A variant of the PSR restricted to entities might require an explanation for the existence and non-existence of entities, or it might be further restricted by requiring a reason only for the existence (or only for the non-existence) of entities. A version of the PSR that is restricted to propositions might range over both contingent and necessary propositions, or it might be further restricted to only one of these sub-domains.