The term ‘Monistic Psychology’ operates across the depth-psychology corpus as both a descriptive label and a polemical target. In its most precise technical deployment — found in Brad Inwood’s reconstruction of Stoic psychology — it designates a theory of action in which reason exercises undivided sovereignty over the soul’s generative processes, leaving no autonomous irrational faculty capable of opposing it. This Stoic monism, Inwood argues, is not a claim that the soul possesses only one power, but that all powers function under the unified governance of reason, producing the notorious paradox of ‘excessive impulse’ that critics from Plutarch to Posidonius found incoherent. In the Jungian and post-Jungian literature, the term migrates into a theological register, where it becomes synonymous with the psychological tendency — identified by Jung himself as characteristic of introversion — to seek a single unifying principle. Hillman, Miller, and Samuels mount their most sustained critique precisely here: the self, monotheism, and monistic psychology are treated as structurally equivalent, all privileging unity over plurality, wholeness over differentiation, the senex archetype over polyvalent archetypal complexity. Jung’s own writings on the monistic tendency acknowledge it as a genuine psychological disposition while simultaneously warning against permitting it to exclude its pluralistic counterpart. The tension between these positions — Stoic, Jungian, and archetypal — constitutes the productive fault line the term marks across the corpus.