Say, non-fictional books. Most popular being the self-help books, but there are others that contains transferrable information; others are very science-y, requiring deeper education to understand. Yet, if any of which, books don't tell you everything, and they sometimes clash with each other.
It's a pain, as the devoted reader tries to take and follow whatever useful for his/her life from reading the book. Throughout the reading, information stays fresh within his/her brain; and it faded gently without application. Unless to fill the time, you're reading a self-help book perhaps to help yourself, or to understand how things work based on life experiences; and perhaps something you could imitate, imitating the success that others have achieved.
We aren't talking about books without enough factual references, nor books that don't research deep enough in the topic that it misleads. We're talking about different authors with different books that have contrasting ideas. One used to be a multi-tasker; and upon reading "Deep Work", one removed two of the three computer screens one haved, and focused on one thing at a time. Perhaps there might be books that wrote about multitaskings? (Though nowadays multitasking is proved a myth, our brain doesn't work that way).
Ok, how about the Outliers book by Malcolm Gladwell that speaks of the 10,000 hours rule? And contrasting it with the Range: Why Generalists Triumph in a Specialized World by David Epstein? It's like a debate, for and against. The latter, as a newer book, will clearly debate against the former; while the former doesn't know his book getting debated for. Irregardless, if you're reading two books that speaks two opposing features, which one are you to follow?
Given the nature of our memory (not everyone is a memory champion, have a memory palace), most probably you would be obsessed with what you read during the reading period. Once it's finished, as we mentioned above, things started to fade. Considering you start to practice it to a habit, following at least some things from the book. Then, you picked up another book that contradicts what you read earlier, how do you choose? Perhaps you don't have difficulty choosing traumas; but one do, and it proves difficult.
Even if it's not the opposing ones, just picking another book immediately after the one you're reading now proves futile. When you brain doesn't have some time to settle, your attention gets diverted to another stuffs, and you forgot what you want to apply. Generally, you only have this much time, and you want to create more than two habits at the same time? Probably doable, though it isn't for most. When you get sucked into the book, or even into the current chapter, you forgot what you read about even in the chapter (if not referencing back to key phrases). Whether it gets retained in long-term memory, or sucked out, isn't clear.
Especially true if they differs. For example, books that clearly speaks one thing throughout the whole book, across chapters, are easier to remember. For example: "The Marshmallow Test", though it has lots of stuffs in, one could only remember one thing: "delay gratification is better than instant gratification". The end. Each chapters speaks towards that single goal. On the other hand, books like "Smartcuts: How Hackers, Innovators, and Icons Accelerate Success", despite being a very good book (One rated it 5 star on goodreads), speaks of different strategies in different chapters. It makes sense though, as the differing strategies, even a few add up to success. It's just not so easy to remember, if you read a few chapters consecutively. While you're in chapter 5, do you still remember what you read in Chapter 1 or 2 without looking back? Maybe not, unless your life needs what is written and you started applying it before continue reading; or unless you have a good memory.
It so happens, to enforce that readers follow what you've written, you don't write opposing observations. Humans (majoritily) work that way: when you're deeply into something, you disregard everything else. Books are designed in such way, when you read, it won't speak of opposing side. Otherwise, it won't be as strong an encouragement when you're with limited knowledge than when you're all knowing. And most people just don't want to disprove themselves. Say, perhaps this article you're reading now is wrong, that what one wrote is wrong; when you write an article yourself, would you want to know in the comments area that what you wrote are wrong? And if your info are wrong, would you allow anyone to debate about it, or close off the comments section so you forever live in your world? Communication allows more learning from differing perspective, yet it feels more confident, more gratifying, when there's no one opposing what you thought. And it feels even more confident, (nose up, head tilt up please), when you're being praised. You feel like you did the right thing, repeated what you spoke that got approval from others in your brain (or speak it out) over and over again (usually while you're alone), and that kept you moving forward. A disproval, on the other hand, means what you just thought out, the brilliant idea, is not so brilliant anymore, perhaps even a trash in the can. In the extreme, you might start punishing yourself for giving wrong information.
Confidence is one thing to move reader forward, but it isn't for information. The act of concealing/hiding information is as displeasing as conveying fake information to readers. Both keep the readers uninformed. The avid readers may snip out missing information when reading; but truly speaking, one don't feel relax when one have to keep one's concentration validating whether what one reads is true or not throught the whole book than just absorbing information and try to find out later. Yet later might be forever, especially when forgotten. Just imagine, if you're reading an English novel (especially the classics) and you're not an first-language-English person: one used to search the dictionary every three words!!! (on average, that's Sherlock Holmes when one was young). How dismotivating it is. Whenever you want to understand a word, you pause to search the dictionary. Similarly, whenever you want to clarify facts, you pause to research about it. Your coffee + book on a lovely evening setting in a calm breeze weather now turns into a busy office work, constantly switching between the book and the laptop (search engines like Google, Bing), as if you're doing scientific research and trying to write a research paper. Unless you already have a knowledge base that what you read contradicts what you know in your brain and its easy to search, others aren't that easy to figure out, while you're reading.
Of course, all these are based on the assumption if you're like one: whom doesn't easily pick up on debating every information one read about, at least not as a habit. One doesn't know how you feel if you are someone whom debate when reading; maybe your brain feels excited and calm even if it's "multitasking" (clarifying is one task, reading is another).
Conclusion
Overall, books are speaking of the pros and missing the cons. It's not a comparative example; and it indeed most doesn't devote a chapter at least for the readers to research about what exceptions the book doesn't apply, or perhaps what the opposite viewers (if more than just black and white perspective, multiple viewpoints) speak about, allowing the readers to decide for himself/herself. At least now, one don't have a good solution to deal with this phenomena unless your book involved in a debate (e.g. your book speaks to disprove another book's information, then probably you can pick up the book that's mentioned and read about it). More often, you're not given the chance to be aware of even its opposing existence.
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