The renowned Greek Mathematician Pythagoras and his followers believed in the existence of numerical patterns in music and the cosmos.
A central belief of Pythagoras and his followers was that "everything is a number."
Numbers: A Threat
A Pythagorean called Hippasus discovered that some quantities couldn't be expressed as the ratio of two whole numbers, like the square root of 2.
These numbers, which are now called irrational numbers, were perceived as a threat and history suggests that Hippasus was executed for his findings.
The Hindu-Arabic number system also included a symbol of zero, contrasting the Roman numerals. When the number system reached Europe, zero made its way to become a negative number, helping with the recording of debt.
As a result, zero was outlawed in Florence in 1299, along with all Hindu-Arabic numbers, because they were believed to encourage fraud.
The resistance to adopting negative numerals persisted for a long time, even seeping into the 19th century. Mathematicians like Gerolamo Cardano avoided using zero, and negative numerals were dismissed as absurd.
Illegal Numbers In The Present
Even today, many numbers are illegal to use. For example, on the anniversary of the Tiananmen Square demonstrations, China banned the search of numbers related to the date of the event (4/6/89) on search engines.
Some numbers are illegal because of what they represent, or their connections to a symbol of political figures. Others could be illegal if they carry protected information such as copyrights, proprietary material, or state secrets.