This is a question about plagiarism and about when citations are necessary.

If one works in a particular academic field there may be statements that everybody in that field accepts, that have been around for a long time, and that it would be a matter of tedious historical research to track back to a published source that can cited: for example, "the number of prime numbers is not finite".

In that specific case I happen to know that I could cite Euclid as a source for that statement, but he did not publish in a peer-reviewed journal, and if I am not focussing on the history of proofs of that proposition, surely it is not plagiarism for me to state that fact without attribution.

That is but one example of a well-known fact.

My question is how does an academic writer decide whether something is so well known that a citation would be ridiculous or whether it would be plagiarism to state it without citing somebody?

  • Type "infinite primes" in Google and at the top right will appear a box stating: "Euclid's theorem is a fundamental statement in number theory that asserts that there are infinitely many prime numbers. It was first proved by Euclid in his work Elements. There are several proofs of the theorem.". I would think that if Google presents the answer itself (not simply as part of the list of matches), it should be considered "common knowledge". – Ray Butterworth Feb 16 at 14:24

Some people will agree with, and some will oppose, the idea that some things are so obvious they don't need to be cited. Here're two essays on Wikipedia about whether one should cite that the sky is blue.

Wikipedia:You don't need to cite that the sky is blue

  • It's pedantic.
  • There is no need to verify statements that are patently obvious.
  • It leads to over-referencing where sentence flow is interrupted by excessive citations.
  • "The purpose of a citation is to guide the reader to external sources where the reader can verify the idea presented, not to prove to other editors the strength of the idea."

Opposing argument: Wikipedia:You do need to cite that the sky is blue

  • Apparently simple facts may be disputed, e.g. the sky isn't always blue, it's black at night.
  • It's reassuring to readers, even if it's obvious.
  • Not everyone has the same "common knowledge". For example it may be common knowledge to Americans how many states there are in the US, but not to the rest of the world.

Suffice to say, there's no consensus on this. General guidelines then would be:

  • If the material is likely to be challenged, cite it.
  • If in doubt, cite it.
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  • +1 for "If the material is likely to be challenged, cite it". "If in doubt, cite it." – Captain Emacs Feb 16 at 15:02
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    Note that there was this paper (in the 90s I think) which reinvented integration. Clearly common knowledge is not always common, it depends on the community. – Captain Emacs Feb 16 at 15:03

Different institutions may have different expectations, but I think this example from Davidson College Library website is fairly typical:

Information qualifies as common knowledge when it can be found in a significant number of sources and is not considered to be controversial. General descriptions of social customs, traditions, and observable world phenomena qualify as common knowledge, as well as popular expressions and sayings such as “the early bird gets the worm.” Common knowledge can vary between subject fields, so think about your audience. If you have doubts about whether something is common knowledge, ask your professor or another expert in the discipline.


Common Knowledge: Davidson College was established in 1837 by Presbyterians.

Needs a Citation: Davidson College was established in 1837 by Presbyterians who bought the land primarily for its rural location, far from the immoral enticements of cities.

For more information about when you don't need to cite, see:

Ballenger, Bruce P. "Appendix A: Guide to MLA Style." The Curious Researcher: A Guide to Writing Research Papers. New York: Longman, 2009. 257-318. Print. Contains information about the circumstances in which to cite and some examples of common knowledge.

Fowler, H. Ramsey, and Jane E. Aaron. "Avoiding Plagiarism and Documenting Sources." The Little, Brown Handbook. New York: Pearson Longman, 2010. 626-634. Print. Contains a section on what qualifies as common knowledge.

Neville, Colin. “What, When and How to Reference.” The Complete Guide to Referencing and Avoiding Plagiarism. 2nd ed. Maidenhead, GBR: McGraw-Hill Education, 2010, 14-27. Ebrary. Web. 06 Jan 2015. Contains numerous examples of when to cite and when a citation is not needed.

In most cases, Euclid's theorem wouldn't need to be cited; anybody with a tertiary education in a maths-adjacent field has probably encountered it. But if you were working in a different context where it might be unfamiliar to many of your readers, then it might be appropriate to cite it.

In the case where you do need to cite it, pretty much any Number Theory 101 textbook ought to be an adequate reference. For statements not likely to be contentious, citations don't always have to be to peer-reviewed sources, and they don't necessarily have to be to the first published appearance of a result.

(Caveat: it is a good idea to avoid chain citations, where you're reporting A's report of B's report of C's publication, but a number theory textbook that includes a proof of Euclid's theorem isn't a chain citation.)

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  • I live your Caveat. Good point. – Earlien Feb 17 at 1:26
  • (downvoter, care to comment?) – Geoffrey Brent Feb 17 at 1:57

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