How people assign mental states to others and how they represent or conceptualize such states in the first place are topics of interest to philosophy of mind, developmental psychology, and cognitive neuroscience. Three competing answers to the question of how people impute mental states to others have been offered: by rationalizing, by theorizing, or by simulating. Simulation theory says that mindreaders produce mental states in their own minds that resemble, or aim to resemble, those of their targets; these states are then imputed to, or projected onto, the targets. In low-level mindreading, such as reading emotions from faces, simulation is mediated by automatic mirror systems. More controlled processes of simulation, here called" enactment imagination", are used in high-level mindreading. Just as visual and motor imagery are attempts to replicate acts of seeing and doing, mindreading is characteristically an attempt to replicate the mental processes of a target, followed by projection of the imagination-generated state onto the target. Projection errors are symptomatic of simulation, because one's own genuine states readily intrude into the simulational process. A nuanced form of introspection is introduced to explain self-attribution and also to address the question of how mental concepts are represented. A distinctive cognitive code involving introspective representations figures prominently in our concepts of mental states. The book concludes with an overview of the pervasive effects on social life of simulation, imitation, and empathy, and charts their possible roles in moral experience and the fictive arts.