Evelyn Underhill published her classic book "Mysticism" in 1911. This is interesting - for what might initially seem an entirely unconnected reason - in that neatly divides the period as between Einstein's "Special Theory of Relativity" in 1905 and his "General Theory of Relativity" in 1916.
In his earlier contributions Einstein particularly focused on the nature of light (electromagnetic energy) and its intimate connections with spacetime. In the latter he probed the mysterious nature of gravity.
Then for the final 35 years or so of his life he turned his attention to the task of properly integrating these two forces in a unified field theory providing the basis for understanding the whole of nature.
As we have seen it is the nature of the integral approach that it establishes clear complementary connections - ultimately in holistic mathematical terms - as between all key physical concepts and their (unrecognised) psychological complements.
So electromagnetic energy and gravity have their psycho spiritual complementary expression equally as the fundamental motivational (or volitional) forces giving purpose to all phenomenal activity. And as we have seen they have been long recognised in the mystical literature as the immanent and transcendent aspects of spirit respectively.
Now why Underhill's work is of considerable interest in this context, is that she probed deeply into process by which unification of these two forces eventually is attained.
Her thesis was quite simple in that she believed that initially the quest for ultimate unification came through the relative dominance of one of these forces (constituting a distinct personality type). So basically she divided aspirants (to ultimate unity) as corresponding either to devotional (immanent) or metaphysical (transcendent) charcteristics.
Indeed she gives many fascinating portraits of these types. It is also interesting - even though she does not explicitly make the distinction - that they can be easily classified under four force headings (allowing for both external and internal expressions in both cases).
So firstly we have those conforming to the (external) immanent type that would be directly associated with the psycho-spiritual expression of the electromagnetic force.
These are often identified as nature mystics who literally see worldly phenomena bathed in a new spiritual light. One good example, which she gives, is the poet William Blake; another is the American Walt Whitman. A recent more modern example of a cognitive - rather than affective kind - is provided through Teilhard de Chardin (who understood the evolution of creation as its gradual transformation towards pure spiritual energy).
Secondly we have others conforming to the (internal) immanent type that would be directly associated (in psycho spiritual terms) with the electroweak force.
Madame Guyon is an oft quoted - by Underhill - extreme representative of this type. She possessed a very unstable emotional temperament leading to frequent shifting states (very reminiscent of the nature of radioactive decay in matter).
More balanced representatives of this emotional type (also mentioned by Underhill) would include Teresa of Avila, Catherine of Genoa and Henry Suso. Initially this type tends to be unduly preoccupied with interior devotional states (thus posing a barrier to true spiritual integration). However in the process of contemplative purification such a personality limitation is overcome to a significant degree.
Thirdly we have the (external) transcendent type that would be directly associated with the psycho spiritual equivalent of the gravitational force. Now getting clear examples here can be difficult as usually both external and internal aspects exhibit themselves to a marked extent!
However Meister Echhart would fit this category due to his clear understanding of the unconditioned dark lying as the source of all created life.
In a less mystical - and more intellectual - sense Einstein himself and the philosopher Hegel would also conform to this type. In this context the intellectual focus is very much on obtaining a coherent global synthesis of the structure of reality (in the very manner that gravity itself provides such structure in global physical terms).
Finally the fourth (internal) transcendent type is very well exemplified (in extreme form) by St. John of the Cross representing the psycho spiritual equivalent to the strong force. With such types the most severe degree of interior purgation is required before contemplative union can be achieved. This leads to a remarkable degree of faith that - literally - binds the personality strongly together.
However perhaps the most important for our purposes here is the Flemish mystic Ruysbroek (who Underhill especially admired).
Almost uniquely among recognised Christian contemplatives, he combined both immanent and transcendent aspects in a wonderfully inspiring manner (thereby giving perhaps the best indication of the true nature of union).
In physics the state where all forces are united as one is often referred to as the supersymmetry of forces. In similar fashion Ruysbroek refers to the highest stage of the contemplative life (where the spiritual union of forces is obtained) as the Superessential Life. He speaks of this - ultimately - ineffable state using some of the most wonderful, joyful and truly inspiring language ever written.
Perhaps we could briefly attempt to summarise this in paradoxical terms as the dazzling darkness (i.e. the intersection point of immanence and transcendence both internally and externally).
So just as this represents the psychological source (and end) of all life, in complementary fashion the source (and end) of all physical life is represented by the intersection point of the four forces where all are united as pure emptiness (yet potential for all life).
And remarkably it is in the same contemplative experience of union that both sets of forces (physical and psychological) are united in complementary fashion as pure emptiness (and potential for creation of all phenomenal form in both a physical and psychological manner).