Not all lives matter.

Brushing your teeth in the morning is a bacterial Holocaust, and you do that at least twice a day (well, you should). You intentionally clap mosquitoes into oblivion, and unknowingly crush other small beings beneath your giant feet every time you stroll through the woods. You probably eat the flesh of animals whose slaughter you indirectly paid for, and plant corpses pass through your digestive system even if you don’t.

The point is we kill… a lot. And we have to! Our lives would not be possible if we were unable to bring about death, and that’s just the way it is. Even after scientists find a way to grow animal and plant matter in labs on a scale large enough to feed all of humanity, the journey towards a world without human-induced death will almost certainly remain unattainable.

The list of organisms we can kill is long, but the list of animals we can’t, from a moral standpoint, is short. The following are the three kinds of animals you can’t kill.

3. The Critically Endangered Animal

There are three northern white rhinos left in the world… and you can’t kill them.

This species, unlike the majority of species, is now a definitively finite resource. Unlike grass, chickens, or ladybugs, we know how many are left, we know it’s not enough to sustain the species’ existence, and we know it’s unlikely we can produce any more.

I pride myself on being reasonable, but I admit that I can’t fully defend this point. It is based on feelings and assumptions, not moral principles. What I mean is, so what if there were no northern white rhinos left? Just don’t kill them, okay?

Anyway, if you are attacked by a northern white rhino, you can totally kill it. Human life is more valuable than endangered animal life, which I will explain in greater detail shortly.

2. The Animal in Custody

I am the proud owner of a sugar glider named Penelope… and you can’t kill her.

You can’t kill her, you see, because she is mine. I own her. I am responsible for her. I pay for her food. I clean her rancid, ammonia-smelling urine off my shirts. I let her bite my fingers and scratch my arms when she wants to play. And I wake up at 3am to see what she’s barking about (and I never know what she’s barking about).

A man’s property and dependents are an extension of himself, and Penelope is an extension of me. For the same reason that I can’t key your car, you can’t squish my beloved marsupial friend… because, in a way, you’d be squishing a part of me too. Her existence and health are a direct result of my actions, and that makes her mine.

This is why it’s illegal to kill a rancher’s cows or your neighbor’s parrot, even if it yammers nonsensically.

However, if Penelope attacks you, do what you have to do to keep yourself safe. She’s really small though, so I doubt it will have to come down to annihilation.

1. The Metacognitive Animal

I am aware of my own thoughts… and you can’t kill me.

When an animal becomes aware of its own thoughts and consciousness, it is no longer an animal. Once it has recognized itself, its ability to conceive, and has in some way or another displayed this capability, it acquires personhood.

And before you say so you think it’s okay to kill babies and the mentally ill, note that this rule applies to all members of the species. Humans have the potential to become metacognitive, and we are not so smart that we can determine a given person’s capabilities, so we have no right to end any human’s life. And if your hamster stood up one day and said dude… I’m a hamster!, we’d have to spread the word that hamster life is now supremely precious too.

I’m not sure if Koko the Gorilla or other individual primates have been proven to be metacognitive. Steven Pinker rejects the concept of ape language, which suggests they probably don’t think anything like the way humans do. But if monkey or other creature metacognition is proven, killing any member of that animal’s species would be wrong.

I think this is self-evident, and it’s why it breaks our hearts to see more intelligent animals like dolphins, elephants, and dogs die. These creatures are closer to the brink of metacognition than most, so we are more empathetic to their doom. And it’s why we don’t feel the same way about seeing a stupid mouse or fish being eaten on a wildlife show.

As applies to the two previous kinds of animals you can’t kill, there is nothing wrong with killing a metacognitive animal that is trying to do serious harm. This is the justification for the Right to self-defense and the defense of others. It’s probably okay to kill a metacognitive animal that presents in imminent threat to a critically endangered animal and an animal in custody too.

In addition to these three kinds of animals, I would say that killing anything without a good reason is wrong. You can swat a fly that’s pestering you, a pig that you want to eat, a tree that looks like it might fall on your house, or a pet that’s become a burden to itself. But you shouldn’t kill an animal just because it’s there or just because you can. That’s brutality. And the ability to abstain from brutality via our metacognitive faculties is what makes us human and makes our lives worth defending in the first place.