Modus ponens is the logical operation which goes like:

• If A then B
• A
• Therefore B

You might see examples like:

Or, in natural language

• If there is a fire, you should leave the building
• There is a fire
• Therefore you should leave the building

I think you have to be very careful with natural language modus ponens. Natural language is not as precise as maths. Usually I see someone start a modus ponens I expect I’m about to be hit with some absolutely fudgey nonsense of the form.

• If unlikely-sounding premise, then my preferred crazy-sounding conclusion
• Unlikely-sounding premise
• My preferred crazy-sounding confusion

Or even

• If a thing has property X, then we should do Y about that thing
• Thing that definitely sounds like it should not have property X has property X
• Therefore we should do Y about the thing that definitely seems like it should not have property X

Usually, I think the culprit is that either the premise is dodgy, or the property X is dodgy, and by “dodgy” I mean poorly specified.

For Example

• If an issue plausibly causes a million times more suffering than anything else in the world, then it’s the only thing worth working on
• Insectg suffering plausibly causes a million times more suffering than anything else in the world
• Therefore we should all be working on insect suffering

There’s two angles to this, the rhetorical angle and the actual logical problems with it.

It’s a useful rhetorical trick. It’s convincing. Why? I think it works so well because we tend to read things in the order they’re presented, to not think very far ahead, and to stick to things we feel we’ve committed to. When we see the first claim, it seems reasonable, and so it gets agreed with and cached. When we see the second claim (plus some evidence) it also seems reasonable, so it gets agreed with and cached. When we see the third claim, we’ve kinda forced ourselves into a corner where there’s friction to go back and disagree with our past selves.

In some ways this is good! Rhetoric is not a dirty word, convincing people of true things is how people end up believing true things.

My issue with this example is that it uses skilled rhetoric combined with faulty reasoning. In this case, the word “plausible” is doing a huge amount of work here. 

Humans have a natural tendency to smooth over ideas into ones which make more sense. In this case, “first_sentence.plausible” means something like “a probability that’s reasonably large”. Meanwhile, “second_sentence.plausible” means something like “we can’t rule this out”.

(As an aside, “plausible” is my personal least favourite EA/rat word and I endeavor to never say it. If you catch me saying it in real life, you can let me know and I’ll squat to the perfect height for you to slap me across the face (or stand on something if you’re that much taller than me).

I think “plausible” also elides a map-territory distinction in a way that “possible” doesn’t: if I can plause something, you can’t really stop me. If I say something is “possible” then you can say “I don’t think it’s possible” and our disagreement becomes clearly factual.)

Asymmetric Weapons

Scott has written that useful “weapons” in arguments are ones which favour the side which is correct. I don’t think this kind of pseudo-logic is asymmetrical, at least when used in this particularly rhetorically-charged way.

I think a better way of presenting it is in a different order:

• I think insect suffering might cause a million times more suffering than human suffering

…and that’s actually the whole interesting part of the claim, which isn’t being placed between two other claims like a trojan horse.

There’s another good way of presenting logic, which is (if I understand correctly) how Nick Bostrom actually used to talk about the simulation hypothesis:

• Spacefaring civilizations might simulate many copies of pre-spacefaring ones
• If they do, we’re more likely to be in a simulation than not
• Therefore we’re more likely than not to be in a simulation
• …That’s pretty crazy. Let’s all sit around and have a chat at whether one of the premises might be broken!

This is, again, more polite. You’re not trying to verbally corner your readers, you’re trying to enlighten them. So be mindful of when you’re being attacked by rhetoric, and equally mindful of when you’re firing off rhetorical shots into a residential neighbourhood.

Editor’s note, this post was written as part of Doublehaven (unaffiliated with Inkaven)

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