The Maharal, a Sage who disputed the Rambam's conception of the soul, claimed that there was a Divine essence that was neither mind nor matter, and that the human soul was one such manifestation of this reality.
He was also bound to admit that it could not be defined.
All we can do is conclude that, just as the Sefirot are emanations from the Godhead and not Divine in themselves, so, too, is the soul a source of all human attributes and not to be identified with any of them.
–Rabbi Adin Steinsaltz
From "The Way of the Soul and Torah: Essence and Structure" in The Candle of God by Rabbi Adin Steinsaltz