Common knowledge is supposed to be the information that is known or is expected to be known by everyone. This can be information widely expected to be known across all of humanity or it can be based on culture, religion, location and group. The idea is important to social life because it can determine a feeling of inclusion or exclusion and isolation or acceptance. It is also related to conventional wisdom.
Philosophers try to distinguish between knowledge and awareness of knowledge. This revolves around the question of whether information is mutually known. For example, Brad has 12 friends and he tells each friend separately that they will meet at a movie theater at 8 p.m. Each of the 12 people knows he or she is going to the movie theater at 8 with Brad, but none know who else knows this information. In this sense, the time and place are common knowledge, but not mutual knowledge.
Some people believe, especially with younger people, that there are certain things everyone should know. Often, such knowledge becomes conventional or received wisdom. This means that the knowledge is not always true. One of the hardest problems for governments and groups is tackling myths, rumors and urban legends.
Sociology plays a part, because knowing or not knowing such information can determine if someone is in a group or excluded from it. Common knowledge can, therefore, be used as a tool for insulting or excluding someone else. Using the Brad example from above, Brad could use the separation method to tell 12 people about the movie trip, but not tell someone else. As the common knowledge is not mutually known, no one will pass the information on to the excluded individual.
Philosopher David Hume was the first to discuss the idea of common knowledge in 1740. David Lewis, however, was the first to introduce the term itself, in 1969. Lewis divided the idea into two distinct types: actual belief and reason to believe. Actual belief is based on actual firsthand experience of something. Reason to believe is based on reading, being told about something or having faith in some kind of knowledge.
In academia, students must cite proof of information stated in an essay or exam answer. This, however, excludes what is deemed to be common knowledge. The names of nations or presidents of America do not need to be cited. More specific dates, quotes and ideas need to be proved.
Common knowledge is used as the basis for a number of quizzes and quiz shows. Information deemed known to most people forms the bulk of easy and medium-level questions in TV shows such as “Who Wants To Be A Millionaire” and “The Weakest Link.” Specialized and uncommon knowledge is used for more difficult questions with higher prizes.