 Thus, a decision about whether such principles hold corresponds to answering a crucial question when building a model for the modal first-order language: what is the relationship between the domains of the different possible worlds? On the one hand, from the perspective of actualism, everything there is (everything that can in any sense be said to be) is actual, that is, it exists; hence, there is a fixed domain across all possibilities. On the other hand, from the perspective of possibilism, ‘the things that exist’ include possible but non-actual objects; hence, different possible worlds might have different domains. There is a large literature on the discussion between these two positions, as the provided references show. (See the entry on the  possibilism-actualism debate for  further discussion.)