Abstract: Discussions of personal identity commonly ignore the question of our basic metaphysical nature: whether we are biological organisms, spatial or temporal parts of organisms, bundles of perceptions, or what have you. This book is a general study of this question. It begins by explaining what the question means and how it differs from others, such as questions of personal identity and the mind-body problem. It then examines critically the main possible accounts of our metaphysical nature. The book does not endorse any particular account but argues that the matter turns on issues in the ontology of material objects. If composition is universal–if any material things whatever make up something bigger–then we are temporal parts of organisms. If things never compose anything bigger, so that there are only mereological simples, then either we are simples–perhaps the immaterial souls of Descartes–or we do not exist at all. If some things compose bigger things and others do not, we are organisms.

Keywords: composition, material object, mereological, metaphysical, ontology, organism, part, personal identity, soul, temporal part