Forbidden knowledge, which is different from 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 publicly citing it is disallowed.
A rich set of examples exist through history.The Roman Catholic church forbids publication of books to which it has not granted Imprimatur.
Throughout the years of isolation in Japan and China all Western literature was forbidden.
Certain 20th century regimes (e.g. communist nations in Eastern Europe and China) placed strong restrictions on foreign propaganda.
In the United States, conservative groups including Jerry Falwell's Moral Majority made several attempts to censor pro civil-rights and feminist works such as Our Bodies, Ourselves.
In many cases this resulted in people defending themselves by creating political jokes. Jokes throughout history have been a powerful instrument to undermine state authority and the public truth associated with it.
Today's (2005) examples in repressive regimes are still uncountable but even liberal societies sometimes impose limits to the freedom of the individual to spread information that is not politically correct. For example, Germany bans the publication of certain books associated with the Nazi regime. 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.
Forbidden knowledge Wikipedia
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 a unified 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 church and state 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.