A more thorough explanation:

Definition: Fictio juris is a legal term that refers to a legal fiction, which is an assumption that something is true even though it may be untrue. It is often used in judicial reasoning to alter how a legal rule operates. It is a device by which a legal rule or institution is diverted from its original purpose to accomplish indirectly some other object.

Examples: One example of a legal fiction is the constructive trust. A constructive trust is a trust that is imposed by a court of equity to prevent unjust enrichment. It is a legal fiction because it assumes that the person holding the property is holding it in trust for someone else, even though there is no actual trust agreement in place.

Another example of a legal fiction is the presumption of innocence. In criminal law, a person is presumed innocent until proven guilty. This is a legal fiction because it assumes that the person is innocent, even though they may be guilty.

These examples illustrate how legal fictions are used to alter how a legal rule operates. They allow the law to achieve its intended purpose, even if it means using a legal fiction to do so.