My Legal Fiction
The simple definition of legal fiction is the presumption of a fact (likely not true) by a court for convenience, consistency, or to achieve justice.
One example of a legal fiction is the universal principal that all people are presumed to know the law; ignorance of the law is not a defense to criminal charges. This was developed so that people cannot avoid punishment for committing illegal acts by claiming ignorance of the law. The premise that all people know the law is a fiction.