There are two kinds of facts that are regarded as common knowledge. Firstly, in academic writing, as a general rule, a fact can be said to be common knowledge, as opposed to expert knowledge when:
- it is widely accessible – that is to say, you may not know the total population of China, but you would be able to find the answer easily from numerous sources;
- it is likely to be known by a lot of people and widely regarded as true or proven;
- it can be found in a general reference resource, such as a dictionary or encyclopaedia.
These kinds of facts require no references. For example: “Pterosaurs were the flying reptiles of the dinosaur age.” ‘Everyone’ knows this, so no citation is needed.
“The more you look at ‘common knowledge’, the more you realise that it is more likely to be common than it is to be knowledge. No real knowledge is common.” – Idries Shah, Reflections
However, “Even the largest pterosaurs may have been able to take off simply by spreading their wings whilst facing into a moderate breeze.” (Wilkinson, M.T., Unwin, D.M. and Ellington, C.P. (2005). High lift function of the pteroid bone and forewings of pterosaurs. Proc. R. Soc. Lond. B.) requires a reference.
Within particular disciplines, the boundaries of what is common knowledge and what is expert knowledge can be ambiguous, especially the further you get into a particular field. However, if it is not common knowledge, you will always need to reference your source.
“Extraordinary claims require extraordinary evidence.” – Carl Sagan
Secondly, there is knowledge which refers to common knowledge outside the academic field that is regarded to be widely known with reference to a particular community.
For someone born in the United Kingdom, it can be considered common knowledge to know that the sea which separates the Britain and Ireland, is called the Irish Sea. However, it probably true to say that this is not common knowledge for someone who has lived his entire life in Tahiti.
For someone born in the United States, it can be considered common knowledge that the US fought both Nazi Germany and Imperial Japan in the Second World War. Whereas this may not be considered common knowledge in North Korea. Having said that, in 2012, according to HBO, four out of ten Americans did not know who the US fought in World War II.
“To know, is to know that you know nothing. That is the meaning of true knowledge.” – Socrates