Derekh Hashem, Part Three, On the Soul and Its Activities

The effect of the separate spirit on the body and the body's effect on it: And behold because of this connection though which this spirit is connected with the body via the lower spirit, it is limited in specific ways. So combination and involvement with the spiritual and ethereal essences are prevented the whole time it is connected with the body - meaning all of the days of a man's life. [Rather,] it is effected by the actions of the body, to be associated through them with the light of the Creator, blessed be He; or to veer away from Him and to cling to the forces of impurity. And preparation for [the ideally] destined perfection, or the distancing from it, is dependent upon this. It also impacts upon the man: It directs the lower soul, guides it and engraves the outlines of understanding, according to its preparation. And it brings out thoughts and the will according to the direction in which it veers.