One of the first concepts one must accept on faith when embarking on a spiritual path is that of a soul being separate and distinct from the physical body that contains it. Not only is this one of the fundamental similarities shared by many of the established religions of the world, but it is also one of the few places where the New Thought movement is wholeheartedly in agreement with the religious establishment. There is much discussion surrounding how and when a soul enters the body during birth, and ultimately what happens to the soul after death, but there is a general agreement that the separation of soul and body is both fundamental and regarded in a positive manner.

However, there is substantially less agreement surrounding the fascinating subject of what happens when more than one soul inhabits a single physical body. Part of the reason for this dissent is the negative connotations associated with the concept of possession. For years, Hollywood horror films have sensationalized the event of a body being taken over against its will, which is akin to a forced metaphysical assault.

The kinder, gentler sibling to possession is known as a walk-in. It is accepted that a walk-in stems from an agreement between the two souls, oftentimes made before the physical body is born. From a human perspective, what this means is a body is "taken over" when the previous inhabitant is finished with his or her time on Earth and is no longer interested in continuing on.