Yeah. It's possible to give quite accurate definitions of some vague concepts, because the words used in such definitions also express vague concepts. E.g. "cygnet" - "a young swan".
What's more likely: You being wrong about the obviousness of the sphere Earth theory to sailors, or the entire written record (which included information from people who had extensive access to the sea) of two thousand years of Chinese history and astronomy somehow ommitting the spherical Earth theory? Not to speak of other pre-Hellenistic seafaring cultures which also lack records of having discovered the sphere Earth theory.
There is a large difference between sooner and later. Highly non-obvious ideas will be discovered later, not sooner. The fact that China didn't rediscover the theory in more than two thousand years means that it the ability to sail the ocean didn't make it obvious.
Kind of a long shot, but did Polynesian people have ideas on this, for example?
As far as we know, nobody did, except for early Greece. There is some uncertainty about India, but these sources are dated later and from a time when there was already some contact with Greece, so they may have learned it from them.
I see no reason to doubt that the article is accurate. Why would Chinese scholars completely miss the theory if it was obvious among merchants? There should in any case exist some records of it, some maps. Yet none exist. And why would it even be obvious that the Earth is a sphere from long distance travel alone?
Nevertheless, I don't think this is all that counterfactual. If you're obsessed with measuring everything, and like to travel (like the Greeks), I think eventually you'll have to discover this fact.
I don't think this makes sense. If the Chinese didn't reinvent the theory in more than two thousand years, this makes it highly "counterfactual". The longer a theory isn't reinvented, the less obvious it must be.
That the earth is a sphere:
Today, we have lost sight of how counter-intuitive it is to believe the earth is not flat. Its spherical shape has been discovered just once, in Athens in the fourth century BC. The earliest extant reference to it being a globe is found in Plato’s Phaedo, while Aristotle’s On the Heavens contains the first examination of the evidence. Everyone who has ever known the earth is round learnt it indirectly from Aristotle.
Thus begins "The Clash Between the Jesuits and Traditional Chinese Square-Earth Cosmology". The article tells the dramatic story of how some Jesuits tried to establish the spherical-Earth theory in 16th century China, where it was still unknown, partly by creating an elaborate world map to gain the trust of the emperor.
They were ultimately not successful, and the spherical-Earth theory only gained influence in China when Western texts were increasingly translated into Chinese more than two thousand years after the theory was originally invented.
Which makes it a good candidate for one of the most non-obvious / counterfactual theories in history.
Without an account of that, IBE is the claim that something being the best available explanation is evidence that it is true.
That being said, we typically judge the goodness of a possible explanation by a number of explanatory virtues like simplicity, empirical fit, consistency, internal coherence, external coherence (with other theories), consilience, unification etc. To clarify and justify those virtues on other (including Bayesian) grounds is something epistemologists work on.
IBE arguments don't exactly work that way. The argument is usually that one person is arguing that some hypothesis H is the best available explanation for the evidence E in question, and if the other person agrees with that, it is hard for them to not also agree that H is probably true (or something like that). Most people already accept IBE as an inference rule. They wouldn't say "Yes, the existence of an external world seems to be the best available explanation for our experiences, but I still don't believe the external world exists" nor "Yes, the best available explanation for the missing cheese is that a mouse ate it, but I still don't believe a mouse ate the cheese". And if they do disagree about H being the best available explanation, they usually feel compelled to argue that some H' is a better explanation.
The problem with calling parts of a learning algorithm a prior that are not free variables, is that then anything (every part of any learning algorithm) would count as a prior. So even the Bayesian conditionalization rule itself. But that's not what Bayesians consider part of a prior.
All of that can be accounted for in a Bayesian framework though?
I mean that those factors don't presuppose different priors. You could still end up with different "posteriors" even with the same "starting point".
An example for an (informal) alternative to Bayesian updating, that doesn't require subjective priors, is Inference to the Best Explanation. One could, of course, model the criteria that determine the goodness of explanations as a sort of "prior". But those criteria would be part of the hypothetical IBE algorithm, not a free variable like in Bayesian updating. One could also claim that there are no objective facts about the goodness of explanations and that IBE is invalid. But that's an open question.
I agree. This is unfortunately often done in various fields of research where familiar terms are reused as technical terms.
For example, in ordinary language "organic" means "of biological origin", while in chemistry "organic" describes a type of carbon compound. Those two definitions mostly coincide on Earth (most such compounds are of biological origin), but when astronomers announce they have found "organic" material on an asteroid this leads to confusion.