In the coming term, the Supreme Court will be considering a pair of cases about state laws which exclude male athletes from female sports. Such laws, which have now been enacted by 27 states, are conceived by their proponents as protecting the integrity of female sports and by their opponents as engaging in invidious discrimination. The cases are to be argued together and are among the highest profile of this SCOTUS term—unsurprisingly, since they concern the constitutionality of central provisions of Title IX, a landmark civil rights law. @John Maier and I have authored an amicus brief on behalf of the petitioners in these cases, with the leading legal scholar and political philosopher @Robert P. George of Princeton as a co-signatory. The brief was also joined by 21 other philosophers, including distinguished professors at Maryland, Virginia, Rutgers, Notre Dame, MIT, USC, NYU, Yale, and Oxford. (Signatories on Twitter: @Tomas Bogardus, @Rona Dinur, @Prof. Gary Francione, @Joel David Hamkins, @Holly Lawford-Smith, @Mary Leng, @Jon Pike, @John Schwenkler, @Brad Skow, @Page about Timothy Williamson, philosopher..) To our knowledge, the brief represents the first time that such a senior group of academics has publicly and explicitly committed to a conclusion on the sex realist side of recent culture wars. According to sex realism, facts about sex play an important role in human thought and action, as they do in the thought and action of other species. Of course, it is not to be assumed that in joining the brief, any signatory’s agreement with sex realism goes beyond its application to the special case of sports, where the approach of separating teams, leagues, events and competitions along the line of sex, as advocated by athletes like @Sharron Davies MBE, @Martina Navratilova and @Riley Gaines, has long enjoyed super-majority support among the broader public. The brief is philosophically substantive. It explains how a general philosophical distinction is relevant to the challenge of how to organize sports. The simplest way to introduce this distinction is by means of examples, like those in the table below. Contemporary philosophers use the vocabulary of “naturalness” to capture the difference between the categories on the left-hand-side of the table—like Gold—and those on the right-hand-side—like Gold or iron pyrite. The former are relatively natural, the latter unnatural. For evocativeness, unnatural categories are often called “gerrymandered,” because their boundaries look weird, jagged, arbitrarily delineated, drawn ad hoc. The brief argues that the vocabulary of naturalness enables a simple explanation of why it is better to organize sports around the line of sex than any other relevant line. The gist is that the sex categories Female (person) and Male (person) are highly natural. Even worse, the opponents of organizing sports around the biological line of sex fail to identify any remotely natural alternative categories to use in their place. Finally, the brief critiques the uncharitable interpretation of the Idaho and West Virginia statutes on which the respondents insist at some length, arguing that if the justice system were required to take challenges of the kind mounted by the respondents seriously in general, its operation would grind to a halt.