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

Question

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?

Answer 1

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.

Answer 2

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.

Example

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.)

Answer 3

This is a false dilemma. You can introduce something without citing it and without passing it for your own work, and so avoid plagiarism. Examples:

It is a standard fact that the sky is blue.

In Section 2, we recall a few well-known facts on prime numbers.

The following results date back to at least Euler.

We recall a few basic facts on prime numbers; these can be found, for instance, on the textbook [1].

Answer 4

I would also consider how much a particular statement is crucial to building your argument.

Research papers build upon prior knowledge. For new results to hold, prior knowledge needs to be trustworthy. That is, for a conclusion to hold, the premises need to hold as well. Critical premises are better to back up with multiple strong sources.

However, research papers are narratives and some statements may be used to increase the readability of the paper. Even if such statements are falsified, the key argument still holds. I would not stress too much if such statements go without strong references.

For example, on a paper comparing atmosphere gasses of different planets you could write:

Earth's atmosphere consists mainly of nitrogen, oxygen, and a mixture of other gasses in small amounts [ref]. To an observer on the surface of Earth, the sky appears blue [no ref]. However, on Mars, the atmosphere is primarily carbon dioxide [ref]. From the surface, the sky appears orange due to dust particles [no ref].

It can be argued whether academic writing can be decorated with statements that do not directly contribute to the main point. The answer varies between fields, venues, and target audiences.