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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.
I once took an online test to see if I would qualify as a contestant for a well-known game show, and I'd say the test was 95% common knowledge questions. People probably know more than they think they do about history and geography and other subjects, and the test creators didn't really try to get more challenging than any other general or common knowledge test I've taken in the past.
I think that every generation has a different definition of common knowledge, however. My generation grew up watching certain TV shows and listening to certain musicians and watching certain movies. To me, common knowledge trivia questions would center around that core group of facts and figures. To my youngest
son, however, that same set of common knowledge would not be as "common".
He grew up with a completely different set of shared cultural experiences with others his age, so their common knowledge definition could never be the same as mine or my grandfather's.
I would say that most of the questions on "Jeopardy" qualify as common knowledge trivia for a person with a four year college degree. Just about anyone who attended college that long should have been exposed to the correct answer during at least one class. Whether or not they can remember those facts under the pressure of the lights and cameras and audiences is another matter.