 The second significant source of concern about Lewis’s conception of fundamental ontology is the role—or rather lack thereof—that modal notions have in it. This concern has two aspects. First, one might hold that some, at least, of the fundamental properties and relations that characterize reality have modal aspects that are ontologically basic. Consider mass: one might hold that it is metaphysically impossible for there to be a world containing just two massive particles, accelerating away from each other—and that this impossibility somehow flows from the nature of mass itself.