Chisholm discussed these problems in a number of papers and chapters. He did not always defend precisely the same combination of views, but certain elements seemed to persist. He steadily maintained a commitment to mereological essentialism for ontologically fundamental physical objects; he tried in various ways to account for facts about persistence, enumeration, and reidentification of ordinary objects by appeal to what he took to be ontologically more fundamental facts about real “substances”. Chisholm’s work on these topics was remarkably provocative. Some have suggested that David Lewis’s development of counterpart theory was intended (among other things) to provide a metaphysical account of the structure of possible worlds that would avoid the strange implications that Chisholm described.