Why is even wrongful possession protected against strangers?

Possession is, in Salmond's words, the most basic relation between a person and a thing — defacto (in fact) physical control, as against ownership which is de jure (in right). Yet law guards possession with a jealousy approaching its protection of ownership itself: "possession is nine points of the law." Why should law protect a mere fact? The answer — peace, evidence and freedom — is the intellectual heart of this unit, and possessory doctrine is a favourite of examiners because it forces the student to separate fact from right.