A more thorough explanation:

Fiction of law is a legal term that refers to 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. This is done by using a device by which a legal rule or institution is diverted from its original purpose to accomplish indirectly some other object.

For example, the constructive trust is an example of a legal fiction. It is a trust that is created by a court to prevent one party from being unjustly enriched at the expense of another party. The court assumes that the trust exists, even though it may not have been created by the parties involved. This allows the court to achieve a fair outcome in the case.

Legal fictions are useful in mitigating or absorbing the shock of innovation, but they can also cause confusion. They satisfy the desire for improvement, which is not quite wanting, at the same time that they do not offend the superstitious disrelish for change which is always present.

Overall, fiction of law is a legal tool that allows courts to achieve just outcomes in cases where the law may not provide a clear solution.