Research has shown that moral judgments depend on the capacity to engage in mental state reasoning. In this article, we will first review behavioral and neural evidence for the role of mental states (e.g., people's beliefs, desires, intentions) in judgments of right and wrong. Second, we will consider cases where mental states appear at first to matter less (i.e., when people assign moral blame for accidents and when explicit information about mental states is missing). Third, we will consider cases where mental states, in fact, matter less, specifically, in cases of “purity” violations (e.g., committing incest, consuming taboo foods). We will discuss how and why mental states do not matter equivalently across the multi-dimensional space of morality. In the fourth section of this article, we will elaborate on the possibility that norms against harmful actions and norms against “impure” actions serve distinct functions – for regulating interpersonal interactions (i.e., harm) versus for protecting the self (i.e., purity). In the fifth and final section, we will speculate on possible differences in how we represent and reason about other people's mental states versus our own beliefs and intentions. In addressing these issues, we aim to provide insight into the complex structure and distinct functions of mental state reasoning and moral cognition. We conclude that mental state reasoning allows us to make sense of other moral agents in order to understand their past actions, to predict their future behavior, and to evaluate them as potential friends or foes.