Let's say you're trying to predict the probability of (H |E)
P(H) = 0,01
Let's say that you've found an instrument lying on the floor with a label on it saying "To be used to measure H). You know nothing about this instrument, neither if it's working or if it's broken, nothing about its hit and miss-rates etc. You can't know if the value it gives to you is reliable or not.
Still, you will have to use it to calculate the probability of (H) given the evidence(E).
Calculate the posterior probability using Bayes theorem.
P(H | E)?
What value would you set on P( E | -H) for this unknown instrument? 0,5?