"Everything in an encyclopedia is Common Knowledge" - actually true?
Taking the statement “anything in an encyclopedia is common knowledge” at face value reveals it is obviously false. In the comments it was pointed out that half of Americans don’t know what century their civil war was in, but there’s also the fact that two thirds of people worldwide have either not heard of the holocaust or engage in holocaust denial. I recently was in a regional trivia competition with thousands of contestants where I had to explain to the organizers that one of their questions were wrong because they had confused density and mass. Virtually all of the questions on Jeopardy! are answerable if you knew everything in an encyclopedia, but even the specially trained contestants don’t know all the answers. Based on these examples alone, I think the idea that the majority of people know all the facts in most encyclopedias is nonsense.
I would strongly advocate for the idea that encyclopedias are irrelevant to the question of if something should be cited. Besides the fact that the vast majority of people don’t know most of the facts in an encyclopedia, there are other reasons to cite things, including:
- Giving credit to the discoverer
- Providing a link between your work and previous work on the subject
- Making it easier for readers to find similar relevant articles.
- Providing accountability for your positions. Sometimes encyclopediae are wrong, and the process of looking for a source would hopefully reveal your mistake.
Even if we pretended that it was common knowledge that every triangle-free graph contains an independent set of size Ω(sqrt(n log n)), all the above reasons to cite that fact would apply.
I'm glad the original poster linked to the Yale Center for Teaching and Learning advice about this, because I could not initially believe this was a standard being put forth.
I might believe that the content of the first paragraph of a general encyclopedia entry might be common knowledge. But the purpose of a citation is to point the user to a source for further knowledge, and/or to allow them to verify that you are reporting accurately. (Stella Biderman's answer is very nice as to the purpose of citing.) Particularly when giving guidance to students, I encourage citing resources that are on the border of being general knowledge, especially if those students are prone to "forget" to cite more generalist sources from which they learned overviews and opinions of a topic, which are sometimes very blatantly echoed in the paper.
Encyclopedias feature useful generalizations, as well as specific facts. I would not require a citation for, "Gold mining is an important part of Mexico's economy," but I would like a citation for, "Mexican mines produce 3.2 billion tons of gold annually." (I completely made up this figure... but it would be nice in such a case to be able to check the source's usage of both "billion" and "ton", whether the measurement is of ore or refined gold, and what year the figure pertains to.)
Why might there be this "encyclopedia" guideline for common knowledge? There are times you don't want writing weighed down by footnotes. In journalism, each claim is (ideally) fact-checked prior to publication. Perhaps in peer-reviewed journals, the theory is that uncited facts are confirmed by the peer reviewers, who would point out that they need a citation if they are not common enough. And perhaps in many rhetoric classes, students appeal to truly common knowledge often enough that instructors do not want them to clutter up their writing with citations for the banal ("America's founding documents proclaim the importance of liberty"; "Facebook is a daily staple for the typical college student").
Finally, I'll argue that the process of confirming "common knowledge" can be generative and improve the paper. When I've sought to use common knowledge in the introduction of my papers, I often looked things up to confirm the key details. (I'm great at trivia, but I "trust, but verify" if I put something in writing.) When I do this, I sometimes stumble across an even more telling anecdote or specific detail that furthers my point, or a helpful resource that explains a core topic in a way students might appreciate. Sometimes I find that the original paper that put forth a now common idea also contains highly relevant extensions that have not been widely developed.
In addition to the good answers already posted, I would suggest that a better way to interpret the contents of an encyclopedia is not as "common knowledge" but as "generally accepted knowledge."
Under this interpretation, it becomes clear that "if you found a statement in the encyclopedia, you don't have to cite it" is not reasonable, as the encyclopedia can be viewed as a sort of mega-review paper. Along those same lines, one will generally find that (as one would expect in a review) the encyclopedia itself is likely to be citing sources for the more specific or specialized information that it contains.
However, one can nicely invert the concept as a rule of thumb: "If you think you don't have to cite a statement, you can confirm by checking an encyclopedia."
In short: if an encyclopedia agrees with you that a citation is no longer necessary, then you should generally feel safe in not adding a citation.