A fined tuned argument.....Heywood style.

[Angrboda]

You think adding more hypotheses increases your prior probability? You are doing bayes wrong, it lowers it. And why do you think just because there are two possiblities on the table, the probabilities are 50% each?

-bolded by me

Principle of indifference.

Which is what you use when you are in a position of maximal ignorance about the alternatives. A probability based on ignorance is meaningless. It doesn't show what the actual probability is, only the maximum we can know from a position of complete ignorance. We aren't concerned with ignorant probabilities but ones based on knowledge. If I want an ignorant opinion on how the universe came to be, I'll ask Bob the grocer. If all you've got backing your probability estimate is ignorance, it's not worth anything.