Psychological essentialism is a prominent view within contemporary developmental psychology and cognitive science according to which children have an innate commitment to essentialism. If this view is correct then a commitment to essentialism is an important aspect of human nature rather than a culturally specific commitment peculiar to those who have received a specific philosophical or scientific education. In this article my concern is to explore the philosophical significance of psychological essentialism with respect to the relationship between the content of our concepts and thoughts and the nature of the extra-cranial world. I will argue that, despite first appearances, psychological essentialism undermines a form of externalism that has become commonplace in the philosophy of mind and language.