  The chief question Lewis faces in this regard is whether there are  enough worlds to do the job. The truth condition   (20)  for the intuitively true   (16)  says that there exists a possible world in which a counterpart of  Algol is no one's pet. By virtue of what in Lewis's theory does such  a world exist? The ideal answer for Lewis would be that some  principle in his theory guarantees a plenitude of worlds, a  maximally abundant array of worlds that leaves “no gaps in  logical space; no vacancies where a world might have been, but  isn't” (Lewis 1986, 86). From this it would follow that the  worlds required by the concretist truth condition for any intuitive  modal truth exist. Toward this end, Lewis initially considers the  evocative principle: 