Forbidden knowledge in contrast to secret knowledge is used to describe forbidden books or other information to which access is restricted or deprecated for political or religious reasons. Forbidden knowledge is commonly not secret, rather a society or various institutions will use repressive mechanisms to either completely prevent the publication of information they find objectionable or dangerous (censorship), or failing that, to try to reduce the public's trust in such information (propaganda). Public repression can create paradoxical situation where the proscribed information is generally common knowledge but publically citing it is disallowed.

A rich set of examples exist through history. Well known to Western readers are forbidden books as defined by the Roman Catholic church in the last few centuries. Through the years of isolation in Japan and China all Western literature was forbidden. Recent (20th century) repressive regimes (e.g. communism in Eastern Europe and China) placed strong restrictions on so called "foreign propaganda". In many cases this resulted in people defending themselves by creating political jokes. Jokes through history have been a powerful instrument to undermine state authority and the public truth associated with it.

Today's (2004) examples in repressive regimes are still uncountable but even liberal societies sometimes impose limits to the freedom of the individual to spread information, which is not politically correct. For example, Germany bans the publication of certain books associated with the Nazi regime, most prominently, Hitler's Mein Kampf. The United States now (after 2001) limits the publication of formerly freely available information which can potentially be related to the production of biological and nuclear weapons.

Sociological and political relevance

Some form of public repression of facts or speculation not desirable to some people or even a majority of the population seems inevitable as societies need to create some common basis of facts to create identity. Critical to political and personal freedom is the level to which this repression is organized through the state or powerful private organizations. Western secular societies have reached the consensus through the late 19th and early 20th centuries that private organizations should not be allowed to engage in compulsory censorship, forcing people to obey their dictates. For example, the separation of state and church in most Western societies mostly prevents religious organizations from repressing individuals based on their personal opinions and beliefs. As well, people are generally allowed to leave employment with a company which may regulate such personal expressions for whatever reason and find employment in less restrictive circumstances. Ideally, political state censorship will be reduced to an absolute minimum in such a society.