“The ten sefirot are the basis: restrain your mouth from speaking, restrain your heart from thinking, and if your heart races return to the Place, thus it is written: running and returning [like the appearance of lightning] (Ezekiel 1:14). And concerning this matter the covenant was made” (Sefer Yetsirah 1:5).
“But sometimes truth flashes out to us so that we think that it is day, and then matter and habit in their various forms conceal it so that we find ourselves again in an obscure night, almost as we were at first. We are like someone in a very dark night over whom the lightning flashes time and time again” (Maimonides, Guide of the Perplexed 4b, cf. BT Sanhedrin 98b).
“Like pondering a thought: the light of that thought suddenly darkens, vanishes; then it returns and shines—and vanishes again. No one can understand the content of that light. It is like the light that appears when water ripples in a bowl: shining here, suddenly disappearing—then reappearing somewhere else. You think that you have grasped the light, when suddenly it escapes, radiating elsewhere. You pursue it, hoping to catch it—but you cannot. Yet you cannot bring yourself to leave. You keep pursuing it” (Rabbi Moshe de León, Sheqel ha-Qodesh 113 translated in Matt, The Essential Kabbalah: The Heart of Jewish Mysticism p. 114).
“[In the center of the fire the likeness of the חַשְׁמַל (ḥashmal), amber (Ezekiel 1:4)]. It has been established that חֵיוָן אֶשָּׁא מְמַלְּלָן (ḥeivan esha memall’lan), sputtering beings of fire, are the splendor that shines, it goes up and down, the fire that is burning. It exists and does not exist, for there is no one who can understand it in one place. No eyes or vision can master it. It is and it is not, in one place and then in another place. It goes up and down. In this appearance is hidden that which is hidden and that which is concealed. This is the mystery called חַשְׁמַל (ḥashmal), amber. Prophets must see, know, and contemplate within this with the vision of the heart and eye more than anything else … All that they contemplate to see and to know is the speculum that does not shine. No prophet merited to contemplate the speculum that shines but Moses, the faithful prophet, for all the keys of the house were placed in his hand. When all the other prophets reached this amber to contemplate with their eyes, their thoughts were confused and the heart was not settled, and because of it they abandoned all corporeal images. Consequently, they saw within what they saw in silence…” (Zohar Ḥadash 38b, adapted from Wolfson, Through a Speculum that Shines: Vision and Imagination in Medieval Jewish Mysticism, cf. Zohar 2:82a).
“In a baraita it is taught: [חַשְׁמַל (Ḥashmal)] עִתִים חֲשׁוֹת עִתִים מִמַללוֹת (itim ḥashot itim mimalot), at times they are silent, at times they murmur—when the utterance goes forth from the mouth of the blessed Holy One they are silent, and when the utterance does not go forth from the mouth of the blessed Holy One they speak” (BT Ḥagigah 13b).