The problem with rejecting generalizations from fictional evidence is that virtually all the evidence we have is fictional.

What do you know about ancient Rome? How do you know it? 
What do you know about World War II? How do you know that? 
What do you know about what's going on right now in a country 500 kilometres away from you? How? 
Are you the son/daughter of your parents? Have you checked? And how have you checked? Do you personally know how to transcribe DNA and have you done so? 
Has the dictator of some state been alive for the last week? Are you sure? 
How many of the laws of physics have you personally tested with hypothesis testing and measuring instruments?

How much of the information you have has you personally tested with your own hands, eyes and head? 
I'm willing to bet, by the sheer number of facts, substantially less than 0.001%.

You are forced to trust what others tell you.

That being said, Arthur Conan Doyle's stories and thoughts put into the mouths of his characters, which he personally thought about for some time before putting into print, have, often, no less predictive/descriptive power than the account of a journalist who describes some event going on somewhere in the world now and gives his assessment of those events. Or, for that matter, that of your friend who recounts what he thinks he just saw. 

"But Sherlock Holmes didn't exist!." Correct, he didn't exist. But there were his prototypes and Conan Doyle's own powers of observation, which are probably no worse (or rather better) than those of most of our contemporaries.

So generalizing from fictional evidence is often better than generalizing from evidence that you don't think is fictional, even though it most often is. Personally, I use examples and advice from ancients who are long gone, Seneca for example. Who knows if he wrote his own letters and followed his own advice. But I don't care, for the advice is good and works, and who gave it is not so important. 

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You are forced to trust what others tell you.

The difference between fiction and non-fiction is that non-fiction at least purports to be true, while fiction doesn't. I can decide whether I want to trust what Herodotus says, but it's meaningless to speak of "trusting" the Sherlock Holmes stories because they don't make any claims about the world. Imagining that they do is where the fallacy comes in.

For example, kung-fu movies give a misleading impression of how actual fights work, not because the directors are untrustworthy or misinformed, but because it's more fun than watching realistic fights, and they're optimizing for that, not for realism.

Kung-fu example is interesting. Let's continue. If you speak about "actual fights" as "actual kung-fu fights" or "actual fights where one of fighters use kung-fu" then how many people saw any of that in real life or know how they are working or participated personally in one of them? And if the number of such people is really low and you do not have one of them as your instructor, then how do you know that your kung-fu class is closer to real fights than those kung-fu movies?

I do not state that kung-fu movies or Sherlock Holmes cites are correct representations of reality (it would be quite strange), I say that the most of other representations (=models of reality) are more or less the same order of magnitude of correctness, and should be considered as such. If you go to kung-fu class with hope that it will help you to fight in the dark alley with 4 thugs, you are in a problem. Your best chance there is to flee and you'd better go to the running club twice a week then. I got a one-handshake experience of that where champion has been beaten hard because of incapacitated friend whom he couldn't leave. 

The stories of SH do not make any claims about the world and nether the less represent some aspects of it quite correct while Herodotus makes such claims and represents it at least skewed and at most completely false, probably honestly mistaken. 

I state that you can personally distill knowledge of reality and useful practical tricks (=more correct model of reality) from fiction books as well as from non-fiction books if you know where to look. 

P.S. real fights ARE fun to watch, you can see it by the number of downloads, but even these videos are usually illegal or hard to find, so the numbers are not very representative

P.P.S. This probably do not stand for current Hollywood production, such as Marvel series, from which there is really few things to distill into knowledge, though even they could be educational in some way.

I do not state that kung-fu movies or Sherlock Holmes cites are correct representations of reality (it would be quite strange), I say that the most of other representations (=models of reality) are more or less the same order of magnitude of correctness, and should be considered as such.

Are you actually suggesting that, for example, General Relativity, or Evolution by Natural Selection, theories with vast amounts of data backing them up, and a litany of verified predictions about the world, should be treated as having "roughly the same order of magnitude of correctness" as a Sherlock Holmes story?

I feel like Isaac Asimov answered this question with far more eloquence than I can muster:

My answer to him was, "John, when people thought the Earth was flat, they were wrong. When people thought the Earth was spherical, they were wrong. But if you think that thinking the Earth is spherical is just as wrong as thinking the Earth is flat, then your view is wronger than both of them put together."

No, I'm not suggesting this, it would be strange. These are things of different domains. Science is the only domain where the knowledge can be verified by some means. E.g. by predictions. 

I'm not the expert in general theory of relativity neither I am the one in theoretical physics, so I cannot speak of these fields with any confidence - how close to reality they are in their current state of development. 

I'm suggesting consider all the information coming to you (except info in the scientific domain which you can and should verify by some means, including personal experiments and network/tree of trusted sources) as generally fictional, and update your beliefs correspondingly. 

Science is the only domain where the knowledge can be verified by some means. E.g. by predictions.

No, that's not true. Science is the only domain which can make causal models about the world. That's a far higher standard than "verifiable knowledge". People can certainly verify historical and biographical events. I can read about, for example, the history of the Roman Empire and know that Hannibal defeated a Roman army at Cannae in the Second Punic War. I can read about Gustavus Adolphus and know that he perished leading a cavalry charge at Lützen on November 6, 1632. What I don't know is the why. Why did the Romans put themselves into a position where they could be enveloped like that? What was Gustavus thinking, leading a charge and putting himself at great personal danger? Furthermore, I don't know what would have happened if things had turned out differently.

That is the difference between science and history. Both tell you what happened. But only science develops detailed theories that explain why and which allow you to make predictions about the world where things would have turned out differently.

Upvoted, but I think there's a fundamental error in this thinking.  "Fiction" is not a binary, it's a gradient (and likely multiple gradients on different dimensions of what updates one might make).

I have repeated some physics and chemistry experiments, and personally experienced a fair amount of things.  Even this is fiction (my memory is imperfect, and in many cases imperfect in non-random biased ways).  But it's LESS fictional than a story or description that was DESIGNED to make me feel or update in a certain way.  In the middle are fairly rigorous descriptions of evidence and inference available in history or engineering books.  

My <mumble>-year old copy of the CRC Handbook of Chemistry and Physics says the boiling point of Helium at one atmosphere pressure is 4.216 degrees Kelvin.  In some sense, this is fiction - I haven't verified it, and I suspect that even the authors are trusting others rather than measuring it themselves.  It's not really helpful to CALL IT "fiction", though.  Intent, expected repeatability, and source all have to factor in to how much you can update on it, and it's far more common to use "fiction" to mean "no claim of correlation to reality".

And on the other side, for standard fiction, one CAN update on the author's values and far-mode beliefs about how things COULD PLAUSIBLY happen.  This isn't evidence of what DID happen, but I've definitely updated my ideas about the variety of human experience and motivation based on work that is acknowledged as fictional.

Agreed with all said. Maybe I wrote it not clearly enough. But where is a fundamental error? I agreed with multidimensionality of any book or for that matter event, and don't think of it as one-dimensional. 

A lot of currently available "non-fiction" gives not more updates of reality than some fiction. 

By the way Mathematics is one of the few fields where you can and have to check everything by yourself from the first principles. In Physics it is harder. 

How much of the information you have has you personally tested with your own hands, eyes and head? I’m willing to bet, by the sheer number of facts, substantially less than 0.001%.

We could go even farther. Why do you trust your own senses? Eyes, ears, smell, taste, and even touch can be deceived. So why believe anything at all?

I trust my senses if I can reasonably be sure that I'm not in the altered state of consciousness. So what? 

Beliefs are important because without them you cannot act in this world. 

My point was that you should not dismiss the thoughts of knowledgeable author what he put into the mouth of some character in the story just because it's "logical fallacy" or "not real" while taking into consideration what your neighbour says on the similar matter. Of course if you are interested in being less wrong.

By excluding from the pool of knowledge of reality on which you update your beliefs any written "fiction" you deprive yourself of valuable chunk of information. This overconfidence is quite common error of rationalists and CFAR participants as I see. 

My point was that you should not dismiss the thoughts of knowledgeable author what he put into the mouth of some character in the story just because it’s “logical fallacy” or “not real” while taking into consideration what your neighbour says on the similar matter.

If Sherlock Holmes tells me that it was raining on "Friday, June 19th 1889"[1], I will trust that information much less than I would trust the report of a historian.

  1. Indeed, June 19th 1889 is a Wednesday, not a Friday

The problem with rejecting generalizations from fictional evidence is that virtually all the evidence we have is fictional.

What do you know about ancient Rome? How do you know it?
What do you know about World War II? How do you know that?

That is not what the word "fiction" means.

Exactly. "Fiction" means not real. But this doesn't mean that you shouldn't update your beliefs on some ideas from it. The problem is that what mainstream considers "non-fiction" has de-facto the same relation to reality as "fiction" but many people quite ready to update their beliefs on it, considering it to being the source of some facts, because this is called "non-fiction", right?