“Rabbi Giddal said in Rav’s name: Any disciple of the wise who sits before his Rav and his lips do not drip bitterness [from dread (Rashi)] they shall be burnt, as is said, his lips are lilies dripping myrrh (Song of Songs 5:13). Do not read מוֹר עֹבֵר (mor over), dripping myrrh, but rather מַר עֹבֵר (mar over), dripping bitterness; do not read שׁוֹשַׁנִּים (shoshanim), lillies, but rather שֶׁשּׁוֹנִים (she-shonim), that repeat [i.e., the lips of those who repeat drip bitterness]? There is no difficulty: This [applies] to a Rav, and that to a disciple. And if you wish, say both refer to a Rav, yet there is no difficulty: The one [means] before he starts; the other, after he starts. Like Rabbah, who before he opened [his lesson] for the Rabbis used to say something humorous and the Rabbis were amused. After that he sat in dread and opened with the import [of his lesson]” (BT Shabbat 30b).
“It has been taught: It is incumbent upon a person who studies wisdom to study some foolishness and know it, because thereby wisdom derives benefit—just as light derives some benefit from darkness, for were it not for darkness, light would not be recognized and the world would gain no benefit from it…. This may be compared to sweet with bitter, for a person does not know the taste of sweet until he tastes bitter. What makes this sweet? You must admit, bitter. This corresponds to what is written: One against the other God has set (Ecclesiastes 7:14)” (Zohar 3:47b).
“There is no worship of the blessed Holy One except from darkness [i.e., doubt], and there is no good except from evil” (Zohar 2:184a).