Total reality = phenomenal realm + noumenal realm. The phenomenal realm is the highly differentiated, external world of material objects as humans experience it with their senses, in other words a representation of the mind in which space, time and causality define the perceptions. The noumenal realm (a single, undifferentiated entity – thing-in-itself – that is spaceless, timeless, non-material, beyond the reach of causality) is inaccessible to experience. According to Schopenhauer, phenomenon and noumenon are the same reality apprehended in two different ways: the noumenon is the inner significance, the true but hidden and inaccessible essence, of what we perceive outwardly as the phenomenal world.
This distinction between phenomenal and noumenal lies at the heart of Wagner's Tristan and Isolde – appearance vs. essence, external vs. internal, day vs. night, beautiful vs. sublime, conscious vs. unconscious, even conventional musical form vs. something beyond, to mention a few of the oppositions. These binary polarities are not inevitably of the either/or variety or the plus/minus of modern parlance. In fact, the boundary between the phenomenal and noumenal realms itself is somewhat gray. There are ways to approach the noumenon from the external world, or even to transcend it; one can think of a black hole as a door to another world. For Schopenhauer this might be accomplished by denial of insatiable desire and the will-to-life. Recognizing the omnipresent character of the will is as close as one can come to knowledge of the world "an sich."
Eric Chafe describes the Schopenhauerian progression in Tristan:
(Chafe, 47)
The confrontation between King Mark and Tristan at the end of Act II encapsulates "many of the key issues of Schopenhauer's philosophy (the essentially tragic nature of existence, the futility of the question "Why," compassion as the basis of morality, the "Grund (reason or cause) " of existence, the limits of what can be experienced, and the impossibility of knowledge concerning the metaphysical noumenon)." (Chafe, 47) Since noumenon cannot cause the phenomenon, King Mark's search for the ultimate causality, the "why" of existence, is in vain.
