Behind the lawsuits and all the rhetoric, the legal clash over transgender rights pitting North Carolina against the Justice Department boils down to a fundamental disagreement over a definition.

Pamela Karlan, who teaches public interest law at Stanford University, summed it up this way:

“The real question here is: How do we determine what a person’s sex is?”

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The 1964 law barring employment discrimination on the basis of “race, color, religion, sex and national origin” says it’s illegal to “discriminate against people based on their sex,” Karlan, who was a senior official of the Civil Rights Division in 2014 and 2015, said in a phone interview. “It doesn’t say you can’t discriminate against people based on the sex on their birth certificate.”

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Karlan said that she would be deeply “disconcerted” about what might happen if transgender people were forced to use the bathrooms of the sex they abandoned.

For example, she said, “what if somebody wearing a mustache and a beard with a suit and tie comes into the (women’s bathroom) bathroom and says, ‘Actually, if you saw the genitalia I was born with, you wouldn’t be upset.’”

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Perhaps, but Karlan said the medical profession has recognized that “people’s sex is made up of a complex group of factors, and for most people, the factors tend to reinforce each other.”

For most people, she said, “their chromosomal sex and their external genitalia and their sense of a social role, and the hormones that are coursing through their bodies all kind of align. For transgender people, their chromosomal sex and external genitalia at birth don’t necessarily correlate with their internal sense of gender.”