> pure opinion (I like green)
"I like green" is a statement of fact about what you like. "Green is good" would be an opinion. It's funny to me how often people mix the two.
> forms can be filtered by these classes fairly easily
Obviously not so easily. :)
opinion is ephemeral, cannot be independently confirmed or denied; purely single viewer information sounds like not-a-fact to me
Opinion is a ghost in the machine—fleeting, unanchored, and vanishing upon inspection. It leaves no footprint for another mind to verify, no independent trail of evidence to confirm or deny its existence. To rely on such solitary perception is to drift in a void where facts do not exist; it is not merely uncertainty, but an absence of reality itself—a whisper that the universe refuses to echo back. --AI Carl Sagan
It seems to me trivial to verify: just ask.
Exactly. A fact is something that can be objectively confirmed or rejected. Exactly what a specific person likes is a fact, because it can be verified/falsified by simply asking them. An opinion, by contrast is 'Elephants are large.' The reason that's an opinion is because while it seems obviously true, it's subjective. Similarly, grass is purple with pink polka-dots is a fact. It's obviously false, but of course right/wrong has nothing to do with fact/opinion.
Opinions can only be seen as facts when modelled as time series data. Verifying an opinion X at time T does not entail X at time T+1.
"Ephemeral" isn't relevant - a statement of fact can be true at one time and false at another.
Confirmation also is somewhat orthogonal: there can be true facts that we can't confirm.
Conventionally, "I like green" is considered a factual claim about the speaker's preferences. Even if you refuse to acknowledge it as a fact because of the restrictions you've given, that doesn't make it an opinion. It's subjective, but it's not an opinion.
Would you say that if someone is experiencing a headache, and they say "I have a headache", that that's also merely an opinion?