"People tend to harbor essentialist beliefs about sex — that is, they presume that each sex category has an underlying "essence" that makes them what they are. This is what leads people to assume that trans women remain "biologically male" despite the fact that many of our sex characteristics are now female. However, there is no “essence” underlying sex; it is simply a collection of sexually dimorphic traits. Some people will presume that sex chromosomes must be this "essence," even though we cannot readily see them, plus there are non-XX or XY variants. Others presume that genitals are this "essence" (probably because they are used to determine our birth-assigned and legal sex), although they can vary too, and may eventually change (e.g., if one undergoes sex reassignment surgery). In day-to-day life, we primarily rely on secondary sex characteristics to determine (or more precisely, presume) what sex a person is — and of course, these traits may change via a simple hormone prescription. Like I said, there is no mystical “essence” underlying sex."