What follows are Nee's 3 definitions. The soul is the place that contains our intellect, emotions, and will. We express to others what is in our spirit through the soul. People are generally more conscious of the outer and outermost man and hardly recognize or understand their spirit at all. Nee uses this passage to show that the Scripture supports his concept that the spirit is encased by the soul.
Wheat has a hard shell on the outside.
As long as the shell is not split open, the wheat cannot sprout and grow. If this shell is not opened, the life inside will eventually die. Nee writes that the point here is not just theoretical but practical. When one is born again there is life inside the shell. Yet unless the outside shell is cracked open to release the life held inside, our spiritual life dries up and eventually dies.