People have a habit of putting everything they encounter into categories. It makes language-based communication possible. If I tell you about the tree at the end of the road you can imagine a scene: you know what a tree is, what a road is, and how the two fit in relation to each other. But you don’t imagine the same scene that I have in mind.

When I write about an older man with a deep, resonant voice, do you think of Orson Welles, James Earl Jones, Richard Burton, Morgan Freeman? They all fit the description, the category. Even though the terms I used are pretty vague: what does “older” mean? Older then you? Than me? Over 50? 60? Showing signs of age such as gray hair or lines on his face?

“Older” must be meaningful in this context or else why would I use it? You see, everybody knows what it means, but you’d not be able to find a consensus, a common definition. Where do you draw the line between older and not-older? It’s a simple binary choice after all.

For argument’s sake, let’s say that older means older than me. That’s easy, right? Oh, you want to know how old I am? Well, I’m not old: I’m middle-aged. So maybe someone who’s close to my age shouldn’t be referred to as older. Let’s change the definition of older to mean an age ten or more years greater than mine. Oh, you still want to know my age?

I guess this is more difficult than it first seemed. Welcome to the territory of the edge case, the point at which we cut from one category to another. The problem arises because our category, older, can’t be defined unambiguously. There is no clear boundary between older and not-older.

That’s not to say that older is useless: it works well as a convenient shorthand, a stereotype. It’s just that the degree to which it applies varies. Morgan Freeman is an older man; George Clooney is less so, Ryan Reynolds probably isn’t, and Daniel Radcliffe is almost certainly not.

And yet older seems such a simple concept. As simple as tree or road. Thinking about trees, you know what a tree is, right? As opposed to a shrub or a bush, or some other plant? And a road: it’s not a freeway, or a track. Right? Or not? There’s some overlap: these terms we use casually, that we understand the meaning of very well, nevertheless are fuzzy around the edges.

We learn the meanings through examples, archetypes. A collection of instances that we are told belong to the category. It’s not about definitions such as you find in a dictionary: those are mere simplified descriptions of what most of us mean when we use a word. When you think about trees you don’t give much, if any, thought to the definition: you just know what a tree is.

It’s the same principle when it comes to gender. You learn about men and women as two mutually-exclusive categories, and you’re taught that everybody fits into one or the other. But just as with any other category, gender has edge cases. There is such a range of variation among humans that there is no characteristic, or set of features, that unambiguously assigns a person to either a male or female gender.

Yes, there are physical characteristics that apply to the majority: these are what are used at birth to decide whether to write M or F on a birth certificate. And for most people that’s fine. But there are edge cases. People who don’t have distinct physical gender characteristics, or people who look like they are male (or female), but are actually female (or male) or neither, or both.

These edge cases, intersex and transgender people, are a minority but not an insignificant one: reliable conservative estimates put the number of transgender people at around 1 in 500, meaning that we account for roughly 15 million people around the world. As a comparison, that’s about the same as the number of Jews worldwide, and more than the population of Greece, Belgium or Sweden.

That’s a lot of people for whom the gender categories don’t properly work. These people exist; gender is an invention. Which do you think might be “wrong”? The sad fact is that there are a lot of people who think of gender, male or female, as absolute binary options: each person has to fit one or the other. Reality doesn’t work that way. It’s not neat or convenient.

Believe me, as a woman myself I’ve tried long and hard to come up with some objective criteria to define womanhood. And for every single characteristic I thought of there are exceptions. It’s not a matter of definitions, it’s a matter of knowing. I have encountered many examples of women in my life and they have shaped my understanding of what a woman is. The same goes for every person who identifies as a woman. It’s an understanding that transcends words. So that is how I know I am a woman.