What is a woman?
In recent years it is a question that has caused political punch-ups, party splits and despatch box spats.
A complex, emotionally-charged and fiercely contested argument around gender, trans rights and women's sex-based rights has often left politicians at Westminster floundering to answer a seemingly straightforward question.
Today's Supreme Court ruling may, just may, calm a political row that has produced all sorts of verbal contortions, particularly from Sir Keir Starmer.
Appearing on the BBC's Question Time election debate in June last year, the Labour leader said he agreed with former Prime Minister Tony Blair's comment that "biologically, a woman is with a vagina and a man is with a penis".
But he was criticised by the Harry Potter author and former Labour donor JK Rowling, who accused the party under Sir Keir's leadership of a "dismissive and often offensive" approach to women's concerns.
The question of self-identification for transgender people has long been thorny for the Labour Party.
Its 2019 manifesto committed Labour to introducing self-identification.
In September 2021, the then-leader of the opposition slapped down one of his own MPs, Rosie Duffield, for saying that only women have a cervix.
Sir Keir told the BBC's Andrew Marr Show: "That shouldn't be said. It's not right."
Duffield has since quit the Labour Party and now sits as an independent.