  Answers that have been given to this question represent a rather deep divide between actualist abstractionists. On the one hand,  “trace” actualists introduce actually existing entities  into their ontologies that can play the role of mere  possibilia in (25) and its like. Trace actualists come in two  varieties: new actualists and haecceitists. New  actualists like Linsky and Zalta (1996) and Williamson (1998, 2000,  2013) argue that, in fact, all individuals are actually existing,  necessary beings but not all of them are necessarily  concrete. Some concrete individuals — those  traditionally (mis-)categorized as  contingent beings — are only contingently concrete. Likewise,  some non-concrete individuals — those, like possible Exotics,  traditionally (mis-)categorized as  contingently non-actual mere possibilia — are only  contingently   non-concrete.[34] 