A fiction is a rule of law that assumes something which is or may be false as true.

Explanation

(English-origin maxim.) A legal fiction is a rule that deliberately assumes something which is or may be false as true, adopted for the purpose of doing justice — it is a tool, not a truth, and may be used only within its proper limits.

Where this applies

• The law treats a corporation as a "person" for purposes of suing and being sued — everyone knows a corporation isn't actually a living person, but the fiction allows the legal system to function.
• A court applies the "mailbox rule" and treats a letter as delivered the moment it's mailed, even though the recipient hasn't received it yet — it's a useful fiction that fixes a clear moment of contract formation.
• The law assumes that everyone knows the law, even though no one actually does — this fiction prevents people from escaping liability simply by claiming ignorance.