“What would happen if suddenly, magically, men could menstruate and women could not?” With this question posed at the outset of her famous essay If Men Could Menstruate, Gloria Steinem imagined a society in which men would make periods an “enviable, worthy, masculine event.” Rather than being seen as an embarrassment or something to hide, “that time of the month” would become a sign of courage in patriarchal societies, she argued. Scientists would prioritize researching the origin of stomach cramps over heart attacks, from which menstruating men would be hormonally protected. All in all, what many societies have traditionally deemed dirty would instantly become an element of pride.

Though fictional, Steinem’s essay allows readers to take in one simple truth: Dirt, and indeed all things perceived as unpleasant, is in the eye of the beholder. What some groups of people consider pure, others will find unclean, as theorized by British anthropologist Mary Douglas. Dirt, she wrote, is simply matter out of place. Blood inside one’s body is normal but finding it anywhere else can prompt squeamishness. A foot may be clean, but putting one’s feet on the dining table is certainly frowned upon. A fingernail isn’t particularly dirty when it’s on a hand, but that changes as soon as said nail ends up on the floor.

Dirt is relative, Douglas points out, because what we consider impure is that which breaks our social order, and different peoples have different sets of rules. In a household, things are dirty when they don’t belong where they are. In a larger social context, however, the implications are far-reaching: Social systems are built around what people consider pure or polluted. Hygiene guidelines, insofar as they are used to determine what is pure, become a way to organize society and control human behavior. Whether religious or not, alcohol bans, rules about women’s premarital virginity, the taboo surrounding periods, or even foods banned from consumption, all contribute to the way people are policed into acting. Needless to say, women and marginalized groups have historically gotten the short end of the stick. Indeed, as Douglas pointed out, the construct of purity is the enemy of change.