In Sickness unto Death, Kierkegaard describes the depressed and the schizophrenic as the two extremes of personality (of course, not using those terms). The touchstone of his insight is not so much dysfunctional behaviors as the existential response of individuals to the realization of finitude, of death and decay.

Kiekegaard sees depression as a philosophical notion as much as psychological, wherein the person sees no solution to death, finitude, the inexorable dissolution of meaning and purpose. These are fundamental observations of what would become existentialism.

On the other end of the spectrum

There is “too much possibility,”

too many visions of and too many calls from what is hidden or manifest below the surface

Yet this universe overwhelms their resources. The self is shattered to the core of identity. This is a classical description of schizophrenia.