I’ve seen this before.

Here, the Times’ Jane E. Brody writes about how we, meaning humans, are mostly bacteria. It’s a cool, little science neato factoid that’s good for at least sparking a “huh!” or getting a kid with a bit of aptitude to say “whoa!” or a first date to, in her head say, “Well, at least maybe he’s not an idiot.”

In other words, it’s good pop-science — nice on the back of a book, or in a colorful placard at a forward-thinking museum.

I don’t know if Katrina Ray, of Nature Reviews, is the original source of the observation, but that’s who Brody references —  saying there are 10 times as many bacterial cells as human in the quote-unquote ‘human’ body, that they constitute a whopping “99.9 percent of the unique genes” in our mortal coils. That is really cool, huh? But I feel like factoids like that have been circulation in the pop-science aether for a while, maybe for as long as I’ve been aware of this stuff — so, like, the ’90s?

So, I’ve heard this before. And what interests me about the factoid now is not the  “Huh!” weirdness of it the first time you hear about it, but rather, the interesting little pathway to get there.

You need all the biological science that went into defining an organism as a collection of units called cells. Then, the drudge work of characterizing and counting. Then the insight to take a look at the percentages, and realize that the bare numbers might necessitate a shift in how you were looking at things, the black background becoming the foreground. Shadow is now subject. The add-on bacterial cells actually outnumber what you thought was the point of the whole deal: the human cells.

So, now, wait–what?–what is this thing we’re dealing with? A human being with bacteria in it, or a bacterial ecostystem with some human bits trapped inside. Now, from the microscope, to basic algebra, you’ve arrived at philosophy: it’s a pretty deep question of identity.

So, simple head counts turn into identity crisis. That’s way more than a number.