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Common Knowledge

Page history last edited by philosophyofsocialcognition 13 years, 3 months ago

Common knowledge




Common knowledge refers the knowledge each member of a group of individuals holds about something in particular without any of them explicitly knowing that they each share that knowledge. It is the mutual knowledge present among a set of people and generally remains implicit and personal. If among a set of students most of them tend to chat and be rowdy in class, it would be because they have the common knowledge that the professor is unresponsive and oblivious to the response of the class. Although this is what all students know, none of them will know that this is why the other students are acting the same way unless one of them says: “This professor doesn’t care what his students do, we can talk all we want!” Although common knowledge usually remains unspoken, each person believes that the other person assumes the same thing as he does. Hence, if everybody knows that Q, everybody knows that everybody knows that Q, and everybody knows that everybody knows that everybody knows that Q, and so on.



Common knowledge constitutes the basis of all social life. Communication and social interaction all rest on some mutual knowledge and understanding.  Furthermore, common knowledge enables us to predict how certain situations will turn out by allowing us to predict what people will think. If I am going to give a speech to a number of important people, I will re-write my notes based on how I think they will like it. These examples show that social interaction involves much more than mere presence, exchange, and communication between people. It involves realizing that we all have a mutual repertoire of knowledge that we share based on the assumptions that we make. We function based on the idea that other people will think like us and will make the same assumptions and predictions as us in various situations. The prisoner's dilemma (PD),  a classical problem in game theory, is a perfect example of how common knowledge is used. Game theory is a theory of choice based on our common knowledge of how other people will act in the game and the rules of the game. If two people are involved in a serious robbery together and each separately get put through an interrogation and offered a series of options, different costs and benefits will be observed based on the response of each and what they think the other will respond.



  • If they both do not admit to committing the crime, they will each have 2 years in jail
  • If only one of them confesses, s/he will get sent for 10 years in jail while the other will be free
  • If they both confess, they will each get 5 years. 


Although having no time in jail is more beneficial, each of them will have to predict what the other person will respond to be able to judge what they will do. Staying silent may only give them 2 years if the other person remains silent. However it may cost them 10 years  if the other person confesses.  From the common knowledge of rationality and the rules of the game, confessing remains the best option.


This notion of common knowledge is also closely tied to folk-psychology, which is our intuitive theory of mind that is used to describe, explain and predict others’ actions  and enable us to interpret the way humans function in a social setting. These concepts form the basis of social cognition.



See also:



-Folk psychology

-Beliefs and Desires in Social Understanding

-Game theory and social cognition

-Social rationality



- Emma Janssen, University of Toronto

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