Has any philosopher shown that it is not?
Aristotle holds - that the soul is the form, or essence of any living thing.
René Descartes holds - that the mind is a nonphysical substance.
If we assume that:
- Spirit is eternal, at least relative to our existence
- and that any Body is known as such by its likeness to the static idea of it.
--> Then we are in the static part of our understanding of the world we live in - the Spiritual and Material sides of it.
As far as I know we can only observe two phenomenas - Materia and Motion. To me it appears logical that Soul is any motion, governed by the laws of Spirit and executed by Materia.