What are we? Despite much discussion in historical and contemporary philosophy, we have not yet settled on an answer. A satisfactory personal ontology, an account of our metaphysical nature, will be informed by issues in the metaphysics of material objects. In the dissertation, I target two prominent materialist ontologies: animalism, the view that we are numerically identical to human organisms, and constitutionalism, the view that we are constituted by, but not identical to, human organisms. Because of the problems that arise (...) from endorsing these ontologies, I instead advance immaterialism, the view that we are essentially immaterial. In Chapter 2, I discuss how animalists must respond to a widely-discussed metaphysical puzzle, the problem of the many. This puzzle prompts some to endorse revisionary ontologies of material objects, and I argue that the animalist cannot appeal to these revisionary ontologies to solve the puzzle as it arises for personal ontology. In addition, solutions that don’t involve a commitment to revisionary ontology will be unavailable to the animalist: I argue that if animalists make use of non-revisionary solutions to the problem, they must abandon the most successful argument for their view. Absent their most successful argument, animalists will need to motivate the view in some other way. Some new arguments for animalism have been proposed, and I argue that they fail to give us reason to endorse animalism over competing ontologies. Without a strong argument, we should not prefer animalism over the other, more attractive, views. In Chapter 3, I show how constitutionalists face a different problem: explaining how the person is not the very same thing as the human organism, despite sharing the very same parts and occupying the very same physical space. We think that the person and the organism are different things because they have different modal profiles – the human organism can survive permanent loss of psychological life, but the person, presumably, cannot. Constitutionalists must then explain what grounds the difference in modal profiles, but such an explanation is hard to come by. This is an instance of the grounding problem, which is notoriously intractable. While the grounding problem is a well-known challenge to constitutional accounts of objects, I demonstrate that this puzzle is even more threatening when applied to persons. Some “solutions” to the problem fail to solve it at all, and solutions that might get the right result for ordinary objects require accepting that there are a multitude of persons where we ordinarily take there to be only one. We should not accept a personal ontology that requires a commitment to that multitude. I argue that the threat of the grounding problem is so great that we must reject the constitutionalist personal ontology. We will see from these puzzles in personal ontology that materialist solutions are either unsuccessful or yield unacceptable consequences. This should prompt us toward considering, instead, immaterialism. According to immaterialism, persons are not material objects, and the immaterialist can then provide solutions to the puzzles that threatened materialist ontologies. In Chapter 4, I outline these immaterialist solutions and show that the puzzles cannot be reinstantiated successfully against the immaterialist. I then discuss different available varieties of immaterialism and argue in defense of my preferred version. Ultimately, I argue that we are simple, immaterial entities that come into existence at the proper functioning of the brain. Endorsing this view of personal ontology permits us to adequately respond to metaphysical puzzles and retain judgments about persons that should be most important to us. In particular, the immaterialist has the resources to avoid the problem of too many thinkers and retain the judgment that there is exactly one person in circumstances where we take there to be just one. The immaterialist also has the resources to plausibly analyze thought experiments, such as cerebrum-swap cases, that threaten materialist ontologies. All things considered, it remains to be seen which personal ontology has the most evidence in its favor. In the context of debates that arise from material object metaphysics, however, evidence weighs in favor of immaterialism. Materialist personal ontologies are saddled with unacceptable responses to metaphysical puzzles, and endorsing materialism about persons requires taking on a very high cost: Either there are far more of us than we ordinarily take there to be, or there are no persons – far fewer of us than we ordinarily take there to be. Some might argue that these are the only acceptable options, so cost be damned. But we cannot afford to be so cavalier about our personal ontology. Instead, I advance immaterialist solutions to puzzles in personal ontology and propose that, in the interest of saving ourselves and everyone we love, we should seriously consider accounts according to which we are immaterial entities. (shrink)