I would hate to appear as a joy-killer -- or rather a spirituality killer -- but I take it that outside of his teenage years, Piaget was not really a dualist. True, he claimed at one point to be a parallelist (concerning the mind-body problem), but that was a bit unclear, and he later admitted that he was in fact a monist (which doesn't leaves much room for spirituality proper).
May I add, incidentally, that Piaget used the term "transpersonal" in his Sociological studies in the sense that the relationship between A and B is transpersonal for C. He took that notion from a Russian author (don't remember whom), and, unfortunately, did not elaborate.
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