Evidance changes your beliefs

Extraordinary beliefs. require extraordinary evidence. And so that’s basically what Bayes’ formula is about.
It’s the mathematical way of saying extraordinary claims require extraordinary evidence. And so if you think I’m an alien, a priori, it’s a very small probability. But then let’s say you get access to one of my blood tests. And in this resource, you get the information that actually my blood is blue and not red like a human.
So then actually your probability of me being an alien should increase dramatically. And but by how much? That’s the question. That’s exactly what Bayes formula answers. It’s like, depending on what you believed before and the data you’ve observed, by how much should you update your prior beliefs into your posterior beliefs.

Citation from Olga Elf