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.