When writing a book about a subject, if all your text is in your own words, and, contains only commonly known, fundamental, well established knowledge about the subject, which can all be seen in tons of resources, do you need to show any references or bibliography at all - even if you looked at other places while gathering that information, but took only the commonly established knowledge?

If you do not list any sources, can someone come to you and say "OK, you didn’t copy anything from me, but what if you looked at my book (which also has that common knowledge), since you do not list any sources"?

I am asking in terms of avoiding a legal issue. Not in terms of "citing is better for your text to be more reliable" in an academic field, etc.... This is an introductory to intermediate level book I am talking about. It is not something that claims to be at an academic level.

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    Does this answer your question? On citing "common knowledge" statements
    – BCLC
    Nov 24, 2021 at 7:25
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    @Snijderfrey It is not a scientific book, but an introductory to intermediate level book about some engineering subjects. anyone can read it, it is written in plain english with minimal to no math. It also has good information for engineers at many places, although sometimes it will be too redundant for them. Nothing at academic level.
    – upstream
    Nov 24, 2021 at 7:52
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    What kind of "legal issue" are you talking about? Citations don't protect against patent infringement or copyright violation. And you can't violate copyright if you're stating common knowledge in your own words, you have to actually copy something.
    – Barmar
    Nov 24, 2021 at 15:32
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    If your book contains only common knowledge - i.e. things every reader already knows - then what is the purpose of the book?
    – kaya3
    Nov 24, 2021 at 17:15
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    @kaya3 ‘common knowledge’ is usually domain specific. For example, it’s ‘’common knowledge’ among (well educated) chemists that you need to be really careful working with metallic lithium, but I would not expect, for example, a carpenter to know that. Also, everyone has to learn common knowledge somewhere, so a decent primer on a subject (which is what it sounds like the OP is trying to write) is still worth writing. Nov 24, 2021 at 21:36

6 Answers 6


See related question Is there such a thing in the academic world as "common knowledge"?

If the knowledge you're looking of really is common knowledge (the example in the linked question is "the sky is blue"), then it's arguable that you don't have to cite it. But it's still only arguable - reasonable people can press the opposite argument that you still should cite that the sky is blue.

Ultimately, when in doubt, cite it.

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    Are you saying people should cite a source for "the sky is blue"? If so, I do not agree.
    – toby544
    Nov 24, 2021 at 15:35
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    @toby544 you wouldn't be the only one to disagree. However, if you look at the answer to the linked question, there are people who think you should cite "the sky is blue". Some of the objections are not trivial, also - e.g. the sky might be blue, but the night sky is black.
    – Allure
    Nov 25, 2021 at 2:24
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    The colour of the sky is culturally conditioned. Ref: Through the Language Glass, Guy Deutscher. It's not as common knowledge as you think it is.
    – TRiG
    Nov 25, 2021 at 15:16
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    @TRiG “The sky above the port was the color of television, tuned to a dead channel.” - William Gibson, Neuromancer. ;)
    – nick012000
    Nov 26, 2021 at 7:21
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    @TRiG Or, for the TLDR version : The surprising pattern behind color names around the world
    – J...
    Nov 26, 2021 at 19:17

Edit: I am asking in terms of avoiding legal issue.

Legally, you just have to not commit copyright infringement, which should be easy by writing in your own words.

But there are still good reasons to include citations. One is to give a pointer for further reading on the topic for readers who are interested.


Some statements that are "commonly known, fundamental," and "well-established" now were not so in the past. For example, the statement "tobacco smoking causes lung cancer" should have a reference (say, this), in my opinion. If I were to make the statement "one is not a prime number," then I would cite, say, this.

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    It would get rather tedious to write a book in this way, as you would have to either a) cite absolutely everything (eG. the earth rotates around the sun, humans die, etc.) or b) draw an arbitrary line regarding what needs to be cited an what not (eG. cite that 1 is not prime but not that the earth rotates around the sun).
    – ddkk
    Nov 24, 2021 at 15:31
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    While true, I'd wager that the corpus of "common knowledge" tends to grow and fairly rarely shrinks. Something that is common knowledge now is likely to be common knowledge 50 years from now. I don't really see the relevance of the fact that the knowledge wasn't common at some point in the past, since nobody will read the text in the past - the knowledge is common now, and very likely will be in the future. I expect it would be rare to omit a citation for common knowledge and then have someone in the future not know what you're talking about. Nov 24, 2021 at 18:46
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    @NuclearHoagie Well, technically....common knowledge shrinks all the time--if you look at people working with vintage sources (for example, sewing patterns) a lot of instructions are simply not included because it's assumed that the reader knows them...because most people did know that stuff when the pattern was written. That said, going too deep with common citations is still silly. We can cite Aristarchus of Samos for "earth goes around sun" but who do you even cite for "sky is blue"? Nov 24, 2021 at 22:16
  • Are we seriously discussing citing "1 is not a prime" in a mathematical paper? Knowledge that has been common for at least seventy years (if not more) should be fair game (and note that all the references after 1910 to 1 being a prime in the paper you linked are highly suspect or very marginal) Nov 25, 2021 at 10:30
  • @DenisNardin, I agree that "1 is not a prime" is not needed to be cited in a mathematical paper. One of my fields is mathematics education, and some students who may wonder why 1 is not a prime number may find my reference useful.
    – JRN
    Nov 25, 2021 at 11:21

Literally, no. Common Knowledge is held in the commons. Generally speaking if you find it in an encyclopedia or in an undergraduate textbook it is likely to be common knowledge, though the specific expression can be covered by copyright.

But there is another reason for citing some things in textbooks. The reader/student may want to know more and go deeper into the things you write.

One alternative is the fairly typical "Further Reading" section at the ends of chapters. Some of these are more than just a bibliography, with a sentence about what the reader might find there to enhance their learning.

You need to avoid plagiarism also, so don't write in a way that the reader might assume that you originated the ideas. Sometimes you want to name some names of the originators of certain ideas. You might also want to give dates of discovery if not complete citations. "Albert Einstein, in 1899, ...".

  • "Albert Einstein, in 1899, ..." --- Not that it really matters, but 1905 or 1915 would probably be a better year than 1899. (This caught my eye because I read most of this book yesterday while waiting roughly 4 hours for work on my car to get done at an auto shop, plus I've read many biographies of Einstein over the past 50 some years.) Nov 24, 2021 at 17:51
  • Einstein's earliest article was published in 1901 (although it's no longer a well-known paper).
    – Tom
    Nov 24, 2021 at 19:42
  • @DaveLRenfro, actually I was referring to the time period in which he developed the insights, not the publication dates of the papers. He pondered the questions that led to special relativity for about ten years.
    – Buffy
    Nov 24, 2021 at 19:44

Your book should be held to the same standard as a Stack Exchange answer in a similar field

I think that the level to which we hold Stack Exchange answers accountable for sourcing facts should be at least the minimum to which we would hold more traditional books. After all, the body of answers for a given site is often seen as encyclopedic in nature and utility for future readers1 and that's roughly what a book is.

Why do we encourage sourcing of fact assertions? For reasons cited in other answers here

  • it helps to build overall credibility of the post
  • it provides interested readers with resources to read further
  • it helps us spot answers that are just making stuff up by their failure to source

I think the first two can also apply to your book.

The book may be likely to be reviewed and recommended (or not) for use in libraries or in teaching, so if it's well-sourced this might contribute to a better review, which may mean more money in your pocket, wider readership and a better result for your publisher, which may contribute to future "book deals".

This is Academia SE and so view-based answers don't always cite sources

I've ironically fabricated one out of thin air below, but for your "introductory to intermediate level book" I recommend you find the SE site that best matches your topic and review the best received-answers and see how they support their assertions of "common knowledge" with sources, and consider this as your floor, your minimum.

1While SE answers collectively are encyclopedic in nature and utility, individually they also should answer the OP's specific question or attempt to "solve their problem". This is universally known as the "SE is both a floor wax and a desert topping" doctrine.


I think you should consider not just whether you need to cite sources, but also whether you should.

There are multiple purposes to cite sources. One is to properly credit ideas, to avoid appearing to take improper credit for those ideas (plagiarize). This is probably the purpose most associated with "need".

Another is to support your own arguments and statements, by either directing your readers to see the grounding of those statements in other places (which can't be recapitulated every time they were mentioned, otherwise every published item would be longer than the one before it and soon would be completely impossible to digest) or at least presenting the appearance that your ideas have other support[1].

Yet another is to connect your readers to additional reading according to their own interests. It may be common knowledge in a given field, but if a reader wants to know more, where should they go next? What if they want to understand why something is true, rather than merely that it is true? What if they want to understand how that piece of knowledge came about?

I think it may be difficult and overly pedantic to include these sorts of citations to every element of very base knowledge in an academic paper, but if your purpose is to introduce a field to your readers it may be worth giving them branches to more reading rather than just setting them out on their own with little more than Google to help. I would not recommend using the absolute bare minimum of citations according to "need", but rather balancing the level of citation with the level of content you present. Keep your audience in mind when writing, no matter what level you write for.

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