Ignotum per ignotius

Ignotum per ignotius means "unknown by unknown". It is used when a person or thing is known only because it is used to explain something else that is equally unknown. For example, if someone were trying to explain how to open a locked door, but they had no idea what kind of lock it was, they might say "you can open it using ignotum per ignotius - try something unknown to open something else that is just as unknown."