Psychological flexibility stands as the central therapeutic construct of Acceptance and Commitment Therapy (ACT), functioning as both the explicit target of clinical intervention and the implicit telos of the entire hexaflex model. Within the depth-psychology corpus, the term denotes not a fixed trait but a dynamic capacity: the ability to contact the present moment fully, to hold thoughts and feelings without being governed by them, and to orient behavior toward values even in the presence of pain or difficulty. Russ Harris articulates this capacity as the coordinated operation of six interlocking processes — defusion, acceptance, present-moment contact, self-as-context, values clarification, and committed action — whose synthesis constitutes what ACT calls response flexibility across emotional, cognitive, and behavioral registers. The corpus reveals a productive tension between flexibility and its shadow: experiential avoidance, cognitive fusion, and behavioral repertoire-narrowing, each of which contracts the space of possible response. Daniel Siegel’s neurodevelopmental work converges on this terrain from a different direction, framing response flexibility as a prefrontal integrative capacity linking attachment history to narrative coherence and interpersonal functioning. The Taoist I Ching tradition, represented by Liu I-ming, contributes a philosophically resonant counterpoint, treating the balance of firmness and flexibility as the cardinal virtue of cultivated practice. Taken together, these voices construct psychological flexibility not as the absence of distress but as the expansion of one’s repertoire of living.