"Why is it patrilineal? That's how it was historically in Anitam and we see no reason to change it. Or why can't reds marry out of caste? Because it's illegal and no one would want to and the children would be red anyway."

"The reds part... Why is it illegal? If it's patrilineal then it only applies to red men, right? Which is bad enough, I guess..."

"Because reds are unclean, and spreading pollution is a really big problem, so you couldn't just go touching reds even if you wanted to."

"Unclean."

He might sound a bit skeptical under the hollowness.

"Aliens might not have it, or might have some other way of handling it. Anyway, I don't think it bothers reds being unclean, so the garbage truck driver wouldn't want his children to do something else."

"We don't. What is this uncleanness?"

"It's an extremely strong aversive reaction to situations in which contagion would be expected - contact with sewage, dead bodies, garbage, and people who are regularly in contact with any of those. At the thought that something you drank was actually raw sewage, or that your mattress is stuffed with decaying corpses in bags, or that reds are touching things, we feel nauseous and panicked and have a hard time functioning normally. Reds follow containment procedures so everyone isn't in a state of disgust and panic all the time."

Felix thinks about this. "That... sounded like it is entirely in people's heads."

" - yes? So are literally all social institutions. Maybe human ones are different?"

"I meant that it doesn't sound like it accurately reflects a fact about the physical world. At cost of making touching a set of people illegal or comparable to something actually dangerous to your health like drinking sewage."

"The fact that everyone would be constantly miserable if procedures somehow collapsed is a fact about the world."

Felix manages to give a fairly unimpressed look under all that misery.

" - do you just not care about that?" Liset asks curiously.

"No? ...I am assuming your civilization invented soap and similar cleaning products?"

"Yes, there are ways to decontaminate if you touch a red."

"I am talking about cleaning the reds. Even if they handle unclean things it doesn't mean they can't wash off later."

"No, touching a red is as disgusting as touching those things directly, even if they've washed."

"Why?"

"Why is anything disgusting? In the environment of evolutionary adaptedness it was a disease risk and now it's hardwired to cause revulsion."

"...I suppose, if it is that hardwired to you guys."

They all nod.

He actually a bit skeptical, but he is not strong enough to argue the point right now.

Then Afen will return to quizzing him about English.

Which he answer absently.

The cloud around the pebble grows and Felix asks if it's okay to open the window so he can divert the humidity away. He can do that without making the house colder.

They are happy to open the window for him.