Cosmetic surgery and cross-sex hormones can’t change us into the opposite sex. They can affect appearances. They can stunt or damage some outward expressions of our reproductive organization. But they can’t transform it. They can’t turn us from one sex into the other. “Scientifically speaking, transgender men are not biological men and transgender women are not biological women. The claims to the contrary are not supported by a scintilla of scientific evidence,” explains Dr. Mayer. Or, as Princeton philosopher Robert P. George put it, “Changing sexes is a metaphysical impossibility because it is a biological impossibility.”

Contrary to the claims of activists, sex isn’t “assigned” at birth—and that’s why it can’t be “reassigned.” As I explain in my book When Harry Became Sally: Responding to the Transgender Moment, sex is a bodily reality that can be recognized well before birth with ultrasound imaging. The sex of an organism is defined and identified by the way in which it (he or she) is organized for sexual reproduction.

This is just one manifestation of the fact that natural organization is “the defining feature of an organism,” as neuroscientist Maureen Condic and her philosopher brother Samuel Condic explain. In organisms, “the various parts … are organized to cooperatively interact for the welfare of the entity as a whole. Organisms can exist at various levels, from microscopic single cells to sperm whales weighing many tons, yet they are all characterized by the integrated function of parts for the sake of the whole.”

Male and female organisms have different parts that are functionally integrated for the sake of their whole, and for the sake of a larger whole—their sexual union and reproduction. So an organism’s sex—as male or female—is identified by its organization for sexually reproductive acts. Sex as a status—male or female—is a recognition of the organization of a body that can engage in sex as an act.

That organization isn’t just the best way to figure out which sex you are; it’s the only way to make sense of the concepts of male and female at all. What else could “maleness” or “femaleness” even refer to, if not your basic physical capacity for one of two functions in sexual reproduction?

The conceptual distinction between male and female based on reproductive organization provides the only coherent way to classify the two sexes. Apart from that, all we have are stereotypes.

This shouldn’t be controversial. Sex is understood this way across sexually reproducing species. No one finds it particularly difficult—let alone controversial—to identify male and female members of the bovine species or the canine species. Farmers and breeders rely on this easy distinction for their livelihoods. It’s only recently, and only with respect to the human species, that the very concept of sex has become controversial.

And yet, in an expert declaration to a federal district court in North Carolina concerning H.B. 2 (a state law governing access to sex-specific restrooms), Dr. Deanna Adkins stated, “From a medical perspective, the appropriate determinant of sex is gender identity.” Adkins is a professor at Duke University School of Medicine and the director of the Duke Center for Child and Adolescent Gender Care (which opened in 2015).

Adkins argues that gender identity is not only the preferred basis for determining sex, but “the only medically supported determinant of sex.” Every other method is bad science, she claims: “It is counter to medical science to use chromosomes, hormones, internal reproductive organs, external genitalia, or secondary sex characteristics to override gender identity for purposes of classifying someone as male or female.”

In her sworn declaration to the federal court, Dr. Deanna Adkins called the standard account of sex—an organism’s sexual organization—“an extremely outdated view of biological sex.” Dr. Lawrence Mayer responded in his rebuttal declaration: “This statement is stunning. I have searched dozens of references in biology, medicine and genetics—even Wiki!—and can find no alternative scientific definition. In fact the only references to a more fluid definition of biological sex are in the social policy literature.” Just so. Dr. Mayer is a scholar in residence in the Department of Psychiatry at the Johns Hopkins University School of Medicine and a professor of statistics and biostatistics at Arizona State University.

Modern science shows that our sexual organization begins with our DNA and development in the womb, and that sex differences manifest themselves in many bodily systems and organs, all the way down to the molecular level. In other words, our physical organization for one of two functions in reproduction shapes us organically, from the beginning of life, at every level of our being.

Cosmetic surgery and cross-sex hormones can’t change us into the opposite sex. They can affect appearances. They can stunt or damage some outward expressions of our reproductive organization. But they can’t transform it. They can’t turn us from one sex into the other.

“Scientifically speaking, transgender men are not biological men and transgender women are not biological women. The claims to the contrary are not supported by a scintilla of scientific evidence,” explains Dr. Mayer.

Or, as Princeton philosopher Robert P. George put it, “Changing sexes is a metaphysical impossibility because it is a biological impossibility.”

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