Two ideas are helpful here.
First is the idea that spirit is eternal, part of the ONE that most of us call God. Spirit, Holy Spirit, Higher Self, Sacred Self, Oversoul — these are all terms that apply. Spirit has all the qualities of the creative source of all that is.
Then there’s a concept in systems theory called “emergent properties.” It describes what happens when a set of elements work together and something entirely new emerges.
As I understand it, based on all kinds of literature and my own “past life regression” experiences, the eternal spirit that we are chooses to take form, to incarnate as a body, for purposes of extending the ALL in new ways through the creative power of Love. As it does so, it encounters many kinds of unexpected experiences which, being eternal and unchanging, has no effect on it, but which may be part of its purpose.
For me, then, what we call soul (as described by Thomas Moore in Care of the Soul, or Gary Zukav in Soul Stories) is what emerges out of the interaction of the spirit with the body. Soul is the temporary means by which our eternal spirit integrates physical experience into its extension of the All. Soul comes into form as the spirit incarnates, and ends after the spirit completes the incarnation process – usually some time after what we call “death”, and sometimes after many “death” and “rebirth” experiences.