 For discussion of the above problem, in a variety of forms (i.e., concerning direct reference or singular propositions), see Adams (1986: 315–22), Fitch (1994), Markosian (2004), Merricks (2012: 64–67, 2015: 170–3), and Ingram (2016). There’s a modal analogue of this problem, facing actualism (roughly, the view that “only actual things exist”), and a rich literature accompanying it. For discussion of this problem, see Adams (1981), Plantinga (1983), Barcan Marcus (1985), Fine (1985), Davidson (2000), Merricks (2012: 64–71, 2015: 173–80), and Speaks (2012).