I'm actually inclined to give this one to aono- introspection, in and of itself, is not a "Spiritualist" domain. Everybody is capable of introspection to some degree.... which is spiritualist.
However, the sort of introspection required to begin unlocking psionic abilities is doubtlessly a step beyond- everyone can be introspective, but not everyone can dedicate their lives to it, to eschewing the world around them in favour of discerning the deepest truths of their own psyche and the nature of reality, to accepting a worldview in which their thoughts are not dictated by the world around them but rather the world around them is a manifestation of their own thoughts.