A frequent justification in casual conversation is that a certain fact is "common knowledge". It often turns out that most people don't actually share this knowledge. Even claims that are widely believed often turn out to be anywhere from only mostly true to the complete opposite of what is actually the case.

Wikipedia editors are strongly encouraged to find reliable sources to support their edits, and to cite them. Citing sources when your edit is challenged by another editor is Wikipedia policy, and any unsourced edits may be removed. For more information, see Wikipedia:No original research and Wikipedia:Verifiability.

There are some claims that many Wikipedians find acceptable to report as fact, without citing any outside sources. This guideline seeks to define when it's a bad idea to do that.

Acceptable examples of common knowledge

Warning: What appears to be common knowledge for some Wikipedians may be unknown to many from other cultures and backgrounds. For clarification, see WP:NOTBLUE.

When to seek professional help

Certain kinds of claims should most definitely not be left to common knowledge without citations:

Should I believe what other editors say?

You should evaluate the testimony of Wikipedians as you would any other primary source. Keep in mind that it can be rude to simply tell someone "I don't believe you" or "I think you are lying" or "You are so biased; no one should believe anything you say." Many people honestly hold mistaken opinions, and no one likes having their beliefs rejected. Many people also don't realize that the experiences of others are different from their own until other people share them, but are perfectly willing to be enlightened if it's done in a civil fashion.

The most diplomatic thing might be for someone to affirmatively say "I don't think that's correct, and here's some evidence from outside sources or my own experience which don't seem to match up with what you wrote".

If you are thinking "that sounds fishy", but don't have any evidence to support your skepticism, say so. Many readers will have the same doubts. If you have a specific reason for doubt, definitely mention it. If not, you can simply ask some questions derived from Wikipedia:Reliable sources. "That sounds odd to me. Can anyone else verify that?" or "If we took a poll of experts in the field, would they all agree with this?" or "Is there a published source we can cite for this?" or "Is there anyone who is not {a supporter of the cause, a member of the cult, etc.} who could confirm this or offer another perspective?"

See Wikipedia:Wikiquette.

Has it always been that way?

Some facts may be so-called "common knowledge" today, but weren't known in the past or weren't obvious. It's a good idea for there to be some explanation of how these facts were discovered, how they have since been confirmed. For example, that the giant ball of fire in the sky is called the sun is an easily verified fact: all you have to do is check a dictionary. The fact that the Earth revolves around the sun is also a fact, but it's far from obvious from simple observation. A link to the history of this scientific discovery would be excellent documentation.

It can be a good idea to explain how things came to be the way they are. The fact that the letter [A] is the first letter of the alphabet is an easily verified fact, which can be looked up in a dictionary. A link to a linguistic reference that explains the origins of the alphabet would be excellent documentation, although a link to confirmation from a dictionary would suffice.

Weasel words

When reporting claims and opinions, so-called "weasel words" tend to crop up, like "some believe", and "others claim", which should always be avoided. Replace the weasel words with names of people, institutions, or publications, and cite the source of your claim. See Wikipedia:Avoid weasel words.

See also