Shabbetai Donnolo, Sefer Hakhmoni, p. 35-36

Jewish Mysticism
This is the import of what is written, “they have no end.” This instructs us that there is no sage in the world who can know, comprehend, and delve into the knowledge of God, to discover the end and to reach the limit of these ten profound [impenetrable] sefirot. If a sage pursues them and seeks in his mind all the days of the world to comprehend them, it will not amount to anything… For a person cannot delve with his mind to pursue in order to know these ten things which are infinitely and endlessly deep.

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