Spiritual (Integration of Rational Stages)
The highest stage of conventional (psychological) development represents the integration of mind and body (i.e. rational with previous primitive stages of development).
This inherently is a spiritual stage.
However there is a fundamental weakness in the conventional psychological treatment. Basically specialisation of conscious development - which accelerates during the rational stages is suited to differentiating and fragmenting reality.
By the same token it is unsuited to the complementary task of integrating and unifying reality which is based on the unconscious. Thus only a limited form of integration - geared to existing social conventions - is possible at the linear level. More meaningful integration must await "higher" levels.
This spiritual stage can be characterised in mathematical terms.
We have already referred to the horizontal number system as (directly) rational and the vertical number system as (directly) intuitive. Now spiritual understanding arises from the dynamic interaction of conscious and unconscious and is maximised where intuition and reason are equally developed. This can be represented by a diagonal line which intersects the plane comprising horizontal and vertical number axes.
We will give this diagonal number system - representing the spiritual - a fascinating mathematical interpretation at a later stage.
Of course in conventional mathematics, reductionism once more abounds. Each earlier number set is simply seen as a subset of a later set. Thus prime numbers are a subset of the natural number system, They in turn are a subset of the integers which in turn are part of the rational numbers.
This reductionist tendency is also very pronounced in conventional psychology, where earlier "lower" stages of development are evaluated from the perspective of the "higher" rational stages.