The Jungian four functions—thinking, feeling, sensation, and intuition—constitute one of the most architecturally significant frameworks in the depth-psychology corpus, serving simultaneously as a clinical instrument, a cosmological schema, and a theory of consciousness. Jung introduced this typology formally in his 1921 Psychological Types, and the library’s major voices—von Franz, Hillman, Beebe, Sharp, Samuels, and Quenk—each extend, qualify, or contest its implications in distinctive ways. Von Franz insists that the fourfold functional structure is not merely a classification scheme but the expression of a deeper archetypal quaternio, an inborn disposition of the psyche to orient itself through fourfold models; she thus cautions against reducing mythological and religious quaternities directly to the functional scheme. Beebe’s contribution is the development of an eight-function model, extending Jung’s four functions into eight function-attitudes by combining each function with its attitudinal complement. Samuels surveys the empirical challenges and the clinical reception of the typology among post-Jungian analysts, noting persistent skepticism yet sustained practical utility. A recurring tension in the corpus concerns the inferior function: its resistance to conscious assimilation, its intimate entanglement with the unconscious, and its paradoxical role as a site of both suffering and psychic renewal. Hillman, writing on the feeling function specifically, approaches it as a moral and relational capacity rather than a mere classificatory type. Together these voices reveal the four functions as a living theoretical problem, not a settled system.