Spirit, like God, denotes an object of psychic experience which cannot be proved to exist in the external world and cannot be understood rationally.
This is its meaning if we use the word “spirit” in its best sense.
Once we have freed ourselves from the prejudice that we have to refer to concepts of external experience or to a priori categories of reason, we can turn our attention and curiosity wholly to that strange and unknown thing we call spirit.
Spirit and Life
Collected Works 8