As elucidated in the esoteric tradition, the soul is composed of ten aspects of being corresponding to the (kabbalistic) Sefirot of the higher worlds:
Chochma, Binah, Da’at, Chesed, Gevurah, Tiferet, Netzach, Hod, Yesod, and Malchut.
The essence of the soul in itself, however, lies beyond our powers of description and beyond our very grasp.
It eludes us entirely and remains forever unknown.
In a certain old-fashioned manner of speaking, it may be likened to the difference between form and content.
And form, in this Aristotelian sense, is not necessarily the outer shape of things but their characteristics or qualities:
The sharpness of a knife is part of its form, its metallic substance is its content.
If one removes all form (or qualifications) from any object, what remains is amorphous matter.
It is the raw material, the basis for the characteristics, that give the object form and meaning.
Similarly, the essence of the soul, like basic substance, is practically out of our range of observation or understanding.
All that we can know is the revealed aspect of the soul, the vessel of its expression, the instrument through which it functions.
Just as the brain is the manifest instrument of the other, wise ungraspable mind; or as the mouth is the instrument of speech, the ear the instrument of hearing, and so on.
We are able to observe the functional organ, not the sense itself.
So, too, with the soul, its essence is beyond us; at best we know its form as revealed by the way the ten Sefirot manifest and express themselves in a particular person.
–Rabbi Adin Steinsaltz
From "The Way of the Soul and Torah" in The Candle of God" by Rabbi Adin Steinsaltz