A sorcerer has two ways to manipulate people:
1) Move things around in the world.
2) Directly influence people's minds.
I'm not going to talk about option 2 because it stops people from being perfect reasoners. (If there's a subset of option 2 that still lets people be perfect reasoners, I'd love to hear it - that might be the most interesting part of the puzzle). That leaves option 1.
Here's a simple model of option 1. Nature shuffles a deck of cards randomly, then a sorcerer (if one exists) has a chance to rearrange the cards somehow, then the deck is shown to an observer, who uses it as Bayesian evidence for or against the sorcerer's existence. We will adopt the usual "Nash equilibrium" assumption that the observer knows the sorcerer's strategy in advance. This seems like a fair idealization of "moving things around in the world". What would the different types of sorcerers do?
Note that if both Bright and Dark might exist, the game becomes unpleasant to analyze, because Dark can try to convince the observer that Bright exists, which would mean Dark doesn't exist. To simplify the game, we will let the observer know which type of sorcerer they might be playing against, so they only need to determine if the sorcerer exists.
A (non-unique) best strategy for Bright is to rearrange the cards in perfect order, so the observer can confidently say "either Bright exists or I just saw a very improbable coincidence". A (non-unique) best strategy for Dark is to leave the deck alone, regardless of the observer's prior. Invisible has the same set of best strategies as Dark. I won't spell out the proofs here, anyone sufficiently interested should be able to work them out.
To summarize: if sorcerers can only move things around in the world and cannot influence people's minds directly, then Bright does as much as possible, Invisible and Dark do as little as possible, and the observer only looks at things in the world and doesn't do anything like "updating on the strength of their own beliefs". The latter is only possible if sorcerers can directly influence minds, which stops people from being perfect reasoners and is probably harder to model and analyze.
Overall it seems like your post can generate several interesting math problems, depending on how you look at it. Good work!
A (non-unique) best strategy for Dark is to leave the deck alone, regardless of the observer's prior
If I were a Dark, I would try to rearrange the cards so they look random to an unsophisticated observer. No long runs of same color, no obvious patterns in numbers (people are bad random number generators, they think that random string is string without any patterns, not string without big patterns, 17 is the most random number, blah blah blah).
(It's possible that the variation of it can be a good strategy even against more sophisticated agents, because ...
This article is going to be in the form of a story, since I want to lay out all the premises in a clear way. There's a related question about religious belief.
Let's suppose that there's a country called Faerie. I have a book about this country which describes all people living there as rational individuals (in a traditional sense). Furthermore, it states that some people in Faerie believe that there may be some individuals there known as sorcerers. No one has ever seen one, but they may or may not interfere in people's lives in subtle ways. Sorcerers are believed to be such that there can't be more than one of them around and they can't act outside of Faerie. There are 4 common belief systems present in Faerie:
This is completely exhaustive, because everyone believes there can be at most one sorcerer. Of course, some individuals within each group have different ideas about what their sorcerer is like, but within each group they all absolutely agree with their dogma as stated above.
Since I don't believe in sorcery, a priori I assign very high probability for case 4, and very low (and equal) probability for the other 3.
I can't visit Faerie, but I am permitted to do a scientific phone poll. I call some random person, named Bob. It turns out he believes in Bright. Since P(Bob believes in Bright | case 1 is true) is higher than the unconditional probability, I believe I should adjust the probability of case 1 up, by Bayes rule. Does everyone agree? Likewise, the probability of case 3 should go up, since disbelief in Dark is evidence for existence of Dark in exactly the same way, although perhaps to a smaller degree. I also think the case 2 and case 4 have to lose some probability, since it adds up to 1. If I further call a second person, Daisy, who turns out to believe in Dark, I should adjust all probabilities in the opposite direction. I am not asking either of them about the actual evidence they have, just what they believe.
I think this is straightforward so far. Here's the confusing part. It turns out that both Bob and Daisy are themselves aware of this argument. So, Bob says, one of the reasons he believes in Bright, is because that's positive evidence for Bright's existence. And Daisy believes in Dark despite that being evidence against his existence (presumably because there's some other evidence that's overwhelming).
Here are my questions:
I am looking forward to your thoughts.