According to Zeami Motokiyo, yūgen is "a profound, mysterious sense of the beauty of the universe ... and the sad beauty of human suffering." It seems supernatural but is not; it is a beauty in this world that is subtle and hints at the beauty that can be found only in the imagination, like that of still trees in a fog, fading from sight. The concept of this aesthetic was prevalent when The Tale of Genji was written in the 11th century.
In The Tale of Genji, by Murasaki Shikibu, yūgen appears in the scenery, the dialogue, and the action. For example, Genji, on his way to visit "the Rokujō lady," and walks "over a reed plain of melancholy beauty" where "the autumn flowers were gone and insects hummed in the wintry tangles. A wind whistling through the pines brought snatches of music to most wonderful effect, though so distant that he could not tell what was being played." The scene is one of peace and subtle beauty, and the tone, one of sadness, in which lies an almost painful beauty. This setting, we are told, is a "melancholy scene" which "set[s] him off" beautifully.
Yūgen also has a quality of being beautiful beyond description; this beauty may even induce tears. Motokiyo speaks of this directly when she writes that "the evening moon burst forth and the figure [the lady] saw in its light was handsome beyond describing."
When Genji speaks to the lady through the blinds, he says, "Thinking to find you here with the holy maidens, / I followed the scent of the leaf of the sacred tree.” "The scent of the leaf of the sacred tree" would--were it meant literally--be a subtle fragrance, barely discernible. As a metaphor, it is equally faint, yet speaks to the pull the lady has on his senses. He cannot help but follow her, even when the scent is almost gone.
Realizing that his lady will go through with her plans and leave, Genji takes her hand and says, "A dawn farewell is always drenched in dew, / But sad is the autumn sky as never before.” His poetry expresses his intense and confused feelings through universally simple yet sad imagery, the very heart of yūgen.
Almost all of the dialogue takes the form of such poems, using natural imagery to hint at the depth of unspoken emotion. A tone of profound sadness overlays the narrative, punctuated by the occasional tears of the characters. This is yūgen: the ability to express the inexpressible through subtlety.