The term 'plane' in the depth-psychology corpus operates across at least three distinct registers, and the scholarly interest lies precisely in how these registers interpenetrate. In Platonic cosmology, as elaborated by Cornford's commentary on the Timaeus, 'plane' designates a geometric primitive: every solid body is bounded by plane surfaces, which are in turn composed of triangles, the irreducible elements of Platonic physics. This geometrical usage grounds the entire Timaean theory of the elements and radiates into Neoplatonic and Yogic cosmologies. In Sri Aurobindo's integral Yoga, 'plane' becomes an ontological stratum — a level of consciousness or being through which the soul passes and to which it may ascend or descend, from the material through the vital to the supramental. This vertical, hierarchical sense is echoed in Eliade's comparative mythology, where the same archetypal gesture is said to be 'projected upon all planes — cosmic, biological, historical, human.' A third, distinctly contemporary usage appears in Daniel Siegel's developmental neuroscience, where the 'plane of possibility' names the state of maximal, open, undifferentiated potential underlying all specific mental events — a kind of psychological ground-of-being rendered in information-theoretic language. Ann Belford Ulanov's Jungian schema of the 'projection plane' and Donna Cunningham's astrological contrast between the material and astral planes further extend the term's reach. Together these usages reveal a persistent depth-psychological intuition that reality is stratified, and that movement between strata — whether geometric, ontological, or therapeutic — is the defining problem of inner life.
In the library
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accessing the plane becomes a sanctuary for the person with unresolved traumatic states. In other words, initial fear may be seen as a low-lying plateau constructed from the developmentally traumatic experiences in which the uncertainty of not knowing was indeed terrifying.
Siegel argues that the 'plane of possibility' — a state of maximal openness and uncertainty — functions therapeutically as a refuge from trauma, though it initially provokes fear in those for whom uncertainty was itself traumatizing.
Siegel, Daniel J., The Developing Mind: How Relationships and the Brain Interact to Shape Who We Are, 2020thesis
we can now label these three distinct positions as the maximal possibilities in the plane, the elevated probabilities in what we will call a plateau, and the actualization of possibility into actuality as a peak.
Siegel introduces the 'three-P framework' in which the 'plane' represents the ground state of maximal possibility, prior to any actualization, forming the foundational stratum of his model of mind.
Siegel, Daniel J., The Developing Mind: How Relationships and the Brain Interact to Shape Who We Are, 2020thesis
every solid must necessarily be contained in planes; and every plane rectilinear figure is composed of triangles; and all triangles are originally of two kinds
Plato establishes 'plane' as the indispensable geometric intermediary between the solid bodies of the physical world and their ultimate triangular elements, making it foundational to the Timaean theory of matter.
just as in the motion picture an image situated behind the observer appears before him on the 'projection' plane of the screen, so contents of the unconscious are primarily 'projected' indirectly as contents of the 'outside world'
Neumann employs the 'projection plane' — both outward and inward — as the structural surface upon which unconscious contents are cast and first encountered as psychic realities.
Neumann, Erich, The Great Mother: An Analysis of the Archetype, 1955thesis
the cyclical recurrence of what has been before, in a word, eternal return. Here we again find the motif of the repetition of an archetypal gesture, projected upon all planes — cosmic, biological, historical, human.
Eliade argues that the eternal return operates simultaneously across all ontological planes, with each plane replicating the same archetypal structure of cyclical regeneration.
Eliade, Mircea, The Myth of the Eternal Return: Cosmos and History, 1954thesis
Any rectilinear plane face is 'composed' of triangles, in the sense that it can be divided up into triangles; and the triangle, as the surface contained by the minimum number of straight lines, is 'assumed' as the irreducible 'element' of all such figures.
Cornford's commentary explicates Plato's derivation of the plane as the geometric mediator between the solid and its triangular constituents, underscoring the reductive logic of Timaean cosmology.
Plato, Plato's cosmology the Timaeus of Plato, 1997thesis
A sound footing in reality is necessary in order to deal with life on the material plane. Surely we wouldn't be here if there weren't valuable lessons for us to learn from it. The Neptunian, however, often wants to cut loose the moorings of this plane to drift and dream.
Cunningham contrasts the material and astral planes as competing orientations in the Neptunian personality, linking the drive to abandon the material plane with addiction, psychosis, and mysticism as equivalent expressions.
Donna Cunningham, An Astrological Guide to Self-Awareness, 1982thesis
there is a downward attachment so strong as to compel the being to hasten at once to a resumption of the physical life because his natural formation is not really fit for anything else or at home on any higher plane.
Aurobindo uses 'higher plane' to designate the ontological strata available to the evolving psychic entity between incarnations, with the soul's fitness for subtler planes determining the trajectory of rebirth.
What we call unconsciousness is simply other-consciousness; it is the going in of this surface wave of our mental awareness of outer objects into our subliminal self-awareness and into our awareness too of other planes of existence.
Aurobindo redefines unconsciousness as passage into other planes of existence, dissolving the ordinary binary of conscious/unconscious into a graduated topology of awareness.
Aurobindo, Sri, The Synthesis of Yoga, 1948supporting
by 'planes' and 'solids' Plato certainly meant square and solid numbers respectively, so that the allusion must be to the theorems established in Eucl. viii, 11, 12, that between two square numbers there is one mean proportional number
Cornford interprets Plato's use of 'planes' as a numerical concept (square numbers), connecting geometric dimensionality to the mathematical theory of proportional means central to the Timaeus.
Plato, Plato's cosmology the Timaeus of Plato, 1997supporting
Since the triangles, not the solids, are Plato's 'elements', this meets Aristotle's objection that not every part of a pyramid or cube is a pyramid or cube.
Cornford clarifies that the plane triangles, not the three-dimensional solids, are the true Platonic elements, defending the theory against Aristotelian critique of non-homoeomerous composition.
Plato, Plato's cosmology the Timaeus of Plato, 1997supporting
the face which serves as base for a solid to stand on; accordingly, as applied to the triangles, it means their faces considered as possible bases for solids
Cornford specifies the technical meaning of the plane face as the base-surface of solids, anchoring the geometric use of 'plane' within the structural hierarchy of Timaean physics.
Plato, Plato's cosmology the Timaeus of Plato, 1997supporting
There are other ways of dividing an equilateral or a square symmetrically into smaller triangles. Why are the half-square and the half-equilateral better than any possible alternatives?
Cornford interrogates Plato's privileging of specific plane triangles as elemental forms, framing the choice as a philosophical decision about the most generative geometric primitives.
Plato, Plato's cosmology the Timaeus of Plato, 1997supporting
the 6 scalenes in the equilateral face of a pyramid can recombine, in pairs, to make three equilateral faces for pyramids or octahedra or icosahedra of the lower grade
Cornford demonstrates how plane triangular elements enable elemental transformation between the regular solids, making the plane face the operative site of Timaean physical change.
Plato, Plato's cosmology the Timaeus of Plato, 1997supporting
the earth is flat, supported by the air; the same applies to the sun, the moon and the other stars; for all these fiery beings are supported by the air on account of their breadth.
Vernant reports Anaximenes' conception of a flat (plane) earth supported by air, situating the cosmological use of planarity within Presocratic debates about the structure of the cosmos.
Vernant, Jean-Pierre, Myth and Thought Among the Greeks, 1983aside