A more thorough explanation:

Definition: Lost or not lost is a provision in marine insurance policies that sets the effective date of the policy to a time before the policy date. This provision applies even if the insured ship has already been lost at the time the policy is executed, as long as neither party knows or has any means of knowing that the ship has been lost.

Example: Let's say a ship owner purchases a marine insurance policy on January 1st, but the ship was actually lost on December 15th of the previous year. However, neither the ship owner nor the insurance company had any knowledge of the ship's loss at the time the policy was executed. In this case, the lost or not lost provision would apply, and the policy would be effective from a date before the ship was lost.

This provision is important because it protects both the ship owner and the insurance company from any losses that may have occurred before the policy was executed, but were not yet known to either party. It ensures that the policy is effective from a date that is fair to both parties, regardless of when the loss actually occurred.