The term ‘First Principle’ in the depth-psychology corpus occupies a peculiar crossroads between metaphysical cosmology and psychological ontology. Plotinus is unquestionably the dominant voice: across the Enneads he elaborates the First as that which utterly transcends Intellectual-Principle and Being, a simplex prior to all multiplicity, all form, all motion, generating the Divine Mind not through deliberate act but through the inexhaustible overflow of its self-sufficiency. The First is not a being among beings but the condition of their possibility — formless, unlocatable, needing nothing, causing everything. Descartes approaches the same problematic from the opposite direction: the First Principle is God as self-caused, causa sui in the positive sense of an inexhaustible essence that requires no external efficient cause. Von Franz, reading Descartes through a depth-psychological lens, ties this to the Trinitarian exclusion of a fourth principle and the resulting fate of the world-soul. Plato’s Timaeus rehearses an older ambiguity, asking whether fire, water, air, and earth can genuinely serve as first principles, or whether they are themselves derivative. Aurobindo repositions the discussion entirely: the First Principle is Supermind or Truth-consciousness, the real creative agency behind Mind and Matter. What unites these disparate voices is the shared recognition that the First Principle must be simpler, prior to, and generative of all derivative structure — a recognition with direct consequences for how depth psychology conceives the ground of psychic life.