The senex archetype occupies a foundational position in the depth-psychological lexicon developed by James Hillman and elaborated within archetypal psychology. Derived from the Latin for ‘old man,’ the senex names not a biographical stage but an a priori archetypal structure governing order, boundary, meaning, teleological fulfillment, and death within the psyche. Hillman’s decisive contribution was to locate the senex in inseparable polarity with the puer aeternus — two faces of a single archetypal configuration rather than sequential life-phases. The senex appears in its positive register as the Wise Old Man, legislator, sage, and keeper of tradition; in its negative register it calcifies into tyranny, melancholy, rigidity, and the death-by-perfection that strangles any complex that has outlived its vitality. Hillman draws the senex’s mythological substrate primarily from Saturn-Kronos, whose dual nature as world-builder and world-consumer models the archetype’s own irreducible ambivalence. A persistent tension runs through the literature: whether the negative senex is an ego-fault amenable to moral correction, or whether it is a prior archetypal disorder beyond the ego’s jurisdiction. Secondary voices — Beebe, Liz Greene, and Kalsched — extend the archetype into typological, astrological, and trauma-theoretical frameworks, confirming its broad clinical and cultural relevance. Hillman’s insistence that puer and senex form a syzygy, not a developmental opposition, remains the pivotal claim around which all subsequent discussion turns.