Abstract

What makes someone the same person over time? There are (at least) two ways of understanding this question: A person can be the same in the sense of being very similar to how they used to be (similarity), or they can be the same in the sense of being the same individual (numerical identity). In recent years, several papers have claimed to explore the commonsense notion of numerical identity. However, we suggest here that these researchers have instead been studying similarity. We develop a novel method that uses simple intuitions about objects to illustrate these two notions of “same person”, and then asks which concept applies to instances of personal change. Across 4 studies (N = 2446), we find that these previously documented intuitions are best understood as reflecting judgments about similarity, not identity (Experiments 1 and 2). We then use this method to explore the situations in which participants do perceive a change in numerical identity. We find that when a person's entire brain (Experiments 3 and 4) or soul (Experiment 4) has been replaced with that of another person, the majority of participants judge that numerical identity has changed. However, we also note that a substantial minority of participants denied that identity had changed, opening new questions about the role of the body in intuitive judgments of personal identity.