What Made Possession "Nine-Tenths of the Law"?

Remember how it bugged you when your brother took something of yours and when you demanded he give it back he would tell you that "possession is nine-tenths of the law"? Where did that come from? And who says it's nine-tenths as opposed to seven-tenths or something else?

I can't provide specific answers to those questions but I think I can shed some light.

In Beacon Lights of History, Vol. 3, by John Lord, the author discusses the concepts of ownership in ancient Rome. First off, at least in early Rome, the father was master of all, and his wife and children could not truly own anything; it was all his. This right of ownership included the family members themselves and the father could kill his son with impunity, if he so desired. Thankfully, over the course of centuries, the Romans moved away from that extreme.

As for property of the non-family-member type, ownership carried the implicit right to transfer the object to another. Two things were required for this to happen. First, the owner must consent to the transfer. Second, the actual delivery must be made. As Lord states, "Movables were presumed to be the property of the possessors, until positive evidence was produced to the contrary."

So clearly, if you had something in your possession, under the law you had the strong presumption that you were the owner.

But it goes beyond that. Lord also explains that the concept of prescriptive easement was in practice way back in those days:

A prescriptive title to movables was acquired by possession for one year, and to immovables by possession for five years. Undisturbed possession for thirty years constituted in general a valid title.

So you may have acquired something through shady means, but if you can hang onto it it becomes yours. There's nothing precise implied in the "nine-tenths" wording, but the gist is clear. You're in a much stronger position trying to hang onto something than trying to take something from someone else.