Jewish Mysticism
[T]he vision is the subtle and pure splendor [seen in accordance with] the comprehension of the one who receives… “their appearance was like lightning this refers to the subtlety and purity of the comprehension of the one who receives… the attribute [or measure] is that which is received by the separate realities [nifradim, i.e., the ontic realm beneath the sefirot], for the prophets saw the attributes in accordance with their comprehension and by means of their ability to receive they expanded their thought… for from that which he comprehends he can recognize that which he cannot comprehend. Therefore the potencies (middot) arose, for language can comprehend only that which comes out from Him, since a man cannot comprehend the potency of the [divine] speech and the letters (middat ha-dibbur we-ha-’otiyyot) but only the potency [of language] itself (middatah be-‘asmah). There is no potency outside the letters. All the sublime potencies are given to be meditated upon (le-hitbonen), for each potency receives from the potency above it, and they are given to Israel, to contemplate from the attribute seen in the heart, to contemplate to the Infinite.

Translated by Elliot Wolfson in Through a Speculum that Shines- Vision and Imagination in Medieval Jewish Mysticism, Princeton- Princeton University Press, 1994.