What makes someone the same across all possible “configurations” of the world? In this paper, mental essentialism—the theory that mental properties, like being in pain or seeing the color red, are essential to identity—is argued to be a plausible answer to this question. Using thought experiments like Mary’s Room, the Zombie argument, and the Inverted Spectrum, I argue that mental properties are experienced uniquely as oneself and are unique to each individual who experiences them. From this, I propose qua-X-ness, a second-order property of being a mental property of an individual X, as a plausible criterion of transworld identity. I also offer a critique of Kripke’s approach by identifying irregularities and counterintuitive answers resulting from his commitments, and demonstrate how mental essentialism resolves them more intuitively, specifically in cases of non-biological consciousness such as artificial intelligence. This work contributes to various areas, primarily metaphysics and linguistics, by posing a novel understanding of metaphysical personhood that lends itself to intuitions about conscious experience and future technologies. This theory may be useful to programmers interested in the ability of artificial intelligence to analyze counterfactual scenarios in order to ensure that an artificial intelligence can adequately think about itself, e.g., if a self-driving car were to wonder about what would have happened had it swerved right instead of left.