Solitude occupies a remarkably contested position within the depth-psychology corpus, functioning simultaneously as pathology, spiritual discipline, developmental necessity, and ontological condition. The tradition distinguishes sharply between solitude freely chosen and isolation defensively imposed: Grof’s formulation — ‘isolation activates the mind, solitude opens the heart’ — captures a polarity that runs through nearly every major voice. Jung, in the Red Book, renders solitude as an interior desert that the ego must traverse in order to encounter the Self, making it structurally prerequisite to individuation; Hollis extends this, arguing that the capacity for solitude is precisely what enables the individuation process to advance. Winnicott frames solitude developmentally as the capacity to be alone in the presence of another — a paradox that marks mature ego-relatedness. Hillman, characteristically, pushes further, treating the archaic loneliness of the daimon as constitutive of individual existence, neither remediable nor wholly unpleasant. Fromm grounds solitude in the discipline of concentration, positioning it as the paradoxical precondition for the capacity to love. Dana’s polyvagal framing adds a somatic register, noting solitude’s deactivating effect on high-arousal states and its role in self-discovery. Across these traditions — Jungian, existential, contemplative, somatic — solitude names the threshold between social embeddedness and interior depth.