People argue bitterly about transgenderism, yet no one truly understands the phenomenon and the prevailing theories about it are filled with errors. One of these errors may be called gender essentialism. Gender essentialism presumes that a person’s biological sex, which is to say the set of objective physical characteristics that determine whether that person’s body is male or female, corresponds to a complementary set of objective psychological characteristics that determine whether that person’s mind is that of a man or a woman. According to a gender essentialist, a trans person is a person whose physical sex and psychological gender conflict. Trans activists reject gender essentialism by arguing that “man” and “woman” are collections of stereotypes, not objective psychological categories. But then, immediately after explicitly rejecting gender essentialism, trans activists often implicitly embrace its spurious link between “sex” and “gender” by concluding that if psychological gender is a collection of stereotypes, then biological sex also must be a collection of stereotypes. So, when trans activists reject gender essentialism they also reject the validity of the gender category “woman”; but in some cases this rejection leads them also to reject the concept of a “female”, which in turn leads them—as a way of avoiding that concept—to use manipulative phrases such as “people who menstruate”. In reality, our best theories of human biology imply that sex is an objective, physical fact about a human that can be explained only in terms of that human’s genes, whereas our best theories of the human mind imply that transgenderism is a psychological phenomenon that cannot be explained in genetic terms. So although biological sex categories are real, and are determined genetically, they have no psychological analogs to which they correspond or with which they can conflict. Because of the uniqueness and profound complexity of a human mind, none of its significant features, transgenderism included, can be understood in terms of fixed sex categories, never mind in terms of the psychological constructs known as “genders”, which are imagined either to correspond to those categories, or to transcend them merely by increasing their number.