Here’s a guest post from the noble Rob Wiblin of 80,000 Hours.  Posted with Rob’s permission.

I’ve periodically read commenters online say that with random unprecedented events (e.g. a total nuclear war) one can’t give meaningful Bayesian probabilities and therefore the probability of e.g. a nuclear war over the next 100 years is 50/50.

Tabarrok got this from many people in response to his series of blog posts on the likelihood of nuclear war. It’s hard to believe these people are serious, but they are and insist on it even when pressed.

I don’t know what university course melted their brain, but evidently one of them did!

The fastest way to show this is wrong is to ask them three probability questions simultaneously:

1. What is the probability of a single total nuclear war over the next 100 years?
2. What is the probability of a single total nuclear war between between 2121 and 2221?
3. What is the probability of one or more total nuclear wars occurring over the next 200 years?

Someone with this philosophy must respond 50/50 to all of them which leads to an internal contradiction.