" incorrect facts that were accepted at the time of writing" it was the beliefs/opinions about the facts that were false, not the facts. The book should have been called "The half-life of factual claims", but that would sell fewer copies. Factual claims and facts are not the same thing.

"That is, for something to be a fact, it must be a claim that can be evaluated as true or false. An opinion is something that is subjective." so "Australia doesn't exist" is a fact (it can be evaluated as false)?

@DikranMarsupial Does it make sense to say: "In my opinion, Australia doesn't exist"? It's a factual claim. Calling it a 'fact' is confusing because, we tend to think of the word 'fact' as a synonym for 'truth'. The problem is that so many 'facts' turn out to be false, or at least not completely true. As a perhaps overused example: "all swans are white" is a factual statement. It didn't become an opinion when black swans were discovered. It was simply wrong. It wasn't a 'lie' either. I think our language fails us a bit here.

"Calling it a 'fact' is confusing because" it was your definition/criteria, I gave an example to show that it can lead to the absurd (which indicates it is a faulty criterion). "all swans are white" has never been a fact, at one time it may have been a "belief" the distinction between "belief" and "opinion" would seem like splitting hairs to me. " I think our language fails us a bit here." it is more a feature than a bug, see my answer for the reason why.

@DikranMarsupial By that logic, there no facts at all, only beliefs.

nonsense the universe is full of facts, we just can't have certain knowledge of what they are, only beliefs. As Lowri points out some of our beliefs are also facts, we just can't be certain which beliefs are facts. For example, the sun is primarily composed of hydrogen and helium is a belief that very likely to be a fact (but there are some that disagree).

@DikranMarsupial How will you ever know if a factual claim is true? Facts are claims. You are talking about truths.

the idea we can have certain knowledge about reality is wrong headed, there is always residual doubt, we have understood this since at least Hume/Kant. Uncertain knowledge is the best we can do. Sometimes that uncertainty is (to all intents and purposes) negligible (e.g. the Sun is made mostly of H and He), sometimes it isn't.

@DikranMarsupial You seem to be making my point for me.

then you don't understand what I have written (I am not saying anything particularly contentious AFAICS - you can only get complete certainty from formal systems like maths and logic). Sorry, I can see this discussion is heading in a direction that is likely to be unproductive, so I will leave it there. I hope my answer explains my position adequately.

@DikranMarsupial OK. It's a fact that my assertion is supported by a dictionary reference though. There are no facts without people claiming them. The idea that there are free-floating 'facts' in the universe isn't supported by logic.

As I pointed out, "Australia does not exist" is a fact according to that definition. When using a dictionary, it is a bad idea to cherry pick the definition that suits your position whilst ignoring the others (and that other dictionaries may have more). Dictionaries are an indication of what may be meant in some context by a word or phrase, and all of the entries need to be considered. Note my answer points out why sometimes we use words in ways that don't mean what the word means when taken literally. Natural language is full of nuance and subtext not captured by a dictionary.

" There are no facts without people claiming them." no, it was a fact that nothing travels faster than the speed of light in 1520 just as it is a fact now.

To a first approximation, you could say that facts are the things we can't do anything about, either because they are in the past (essentially everything is) or they are bigger than us, like physics. Opinions shrink when we realize that almost nothing is actually in our control, and we don't need to argue over things we can't control. The world is round, or flat. Ok. Curious that it appears to be round from high enough up...

@DikranMarsupial If you really didn't believe Australia exists and claim seriously claim it doesn't exist, then in your mind, it would be a 'fact'. That doesn't make it objectively true, though. Essentially, there's no difference between a 'fact' and a 'factual claim' other than whether you believe it is true.

@JimmyJames The dictionary definition does not say it is a fact in your mind, it just says a fact is "a piece of information presented as having objective reality". It is equivalent to a flat Earther stating that the Earth is flat. That would then be a fact according to that dictionary definition. If you are unhappy with that, perhaps you need to consider other definitions available in dictionaries, such as "that which actually exists or is the case; reality or truth:" (dictionary.com/browse/fact) - which is what it usually means in a philosophical context.

What is the difference between something that is a fact in your own mind and your opinion?

@DikranMarsupial It's a little unclear what you are asking or why it isn't addressed in my answer but a 'fact' 'in my mind' is a something that I believe to be true (or reasonably true) about objective reality, at a given moment e.g. right now, I believe that the surface of the earth is spheroid in nature. An 'opinion' 'in my mind' is something that I think but it is subjective such as Nirvana was a better band than Candlebox. You might think Candlebox is better but that's not an argument about objective reality, that's about taste and preference.

@ScottRowe I'm not sure what your comment has to do with my answer, much less whether you are in agreement or disagreement. I am absolutely interested in any respectful critique or questions you might have, however.

@DikranMarsupial "The dictionary definition does not say it is a fact in your mind, it just says a fact is 'a piece of information presented as having objective reality'. It is equivalent to a flat Earther stating that the Earth is flat. That would then be a fact according to that dictionary definition. If you are unhappy with that ..." I'm perfectly happy with that as is it exactly what I am arguing. Why do you think I am unhappy with my own argument? Do you think I am insane?

I apologize for lack of clarity. I was going with your statement "a fact ... must be a claim that can be evaluated as true or false" and extending it with the assumption that it has been evaluated as true already, and that we cannot alter it. For example, the light is on. Ok, but I can flip the switch and turn it off, so it is not a very significant fact. The earth goes around the sun is more important and useful, most likely. But I am struggling to understand the subtlety of your statements (truly) and am interested to understand.