The example of seeing a stripy equine in Colorado and having to decide whether it's a horse, ass, or zebra is a good one and highlights what base rate means. H

The example of seeing a stripy equine in Colorado and having to decide whether it's a horse, ass, or zebra is a good one and highlights what base rate means. However, it even more strongly highlights a problem that comes up in probability: missing assumptions. As in, there's several important missing assumptions in the example. Suppose you're hiking in a country without wild zebras and you see what is clearly a white equine with black stripes. Which is more likely: 1. It's a zebra. 2. Someone painted stripes on their horse 3. You saw patterns of shadows on a white horse 4. You accidentally absorbed hallucinogenic fungus through your skin when you wiped you hand on a tree branch ...and so on. The longer you think about any unlikely events, the more unlikely possibilities you can come up with. In fact 2. is something I've personally encountered on a hike before, although it wasn't in Colorado. How would you know which of these unlikely events had a higher base rate? It seems unknowable. It depends on whether there are some isolated farms nearby where people keep horses, or whether there was a small zoo in the next town that you didn't know about, whether hallucinogenic fungus grows in this area, and so on. I think it's important to address this in any discussion of probability theory, because hard though the maths is, the complexity of reality is even harder and it's usually omitted or hand waved away when teaching this stuff.