Within the depth-psychology corpus, ‘Part’ operates on at least three distinct registers that rarely converge but illuminate one another. In Richard Schwartz’s Internal Family Systems model — the most architecturally developed deployment of the term — parts are theorized as ‘discrete, autonomous mental systems, each with their own idiosyncratic range of emotion, style of expression, abilities, desires, and views of the world,’ making them far more than mood-states or cognitive habits. The IFS literature insists on a quasi-personological ontology: parts are inner persons, often age-regressed, capable of fear, hope, love, and transformation. Kalsched’s Jungian work treats analogous internal figures — protector-persecutors, self-care systems — as autonomous complexes with archetypal underpinnings, overlapping substantially with Schwartz’s taxonomy even without sharing its vocabulary. Van der Hart’s structural dissociation model imports the term into trauma theory, distinguishing parts of the personality (ANP/EP) as functional sub-systems shaped by evolutionary action systems. Against these clinical frameworks, the Platonic tradition raises the question of mereological coherence: in Plato’s Parmenides, parts entail a whole to which they belong, and in the Republic the soul is itself partitioned into rational and non-rational strata. These philosophical antecedents underwrite the therapeutic intuition that integration — not elimination — of parts is the goal of depth work.