How does the soul relate to the spirit?

To neglect our body is to impoverish our spirit. 

The spirit body partakes both of the mental and bodily food which we imbibe and it follows that if there is a spiritual counterpart to the physical body, there must be a spiritual counterpart to the food which we eat, and this latter enters the spirit body in the same way that the physical body is fed by the material food. We can imagine that this proves that we are wrong in saying the spirit body is fed by the mind and not by the material body. There is this difference—that whereas the material food we take into our bodies is needed to replace waste of the physical structure, the spiritual counterpart of the body imbibes the food for the purpose of preventing the waste of spiritual matter. Our spiritual body would waste away just as our material body would if we did not replace the waste by the food which the physical organism absorbs and of which the spiritual body receives its counterpart. Thus we see the spiritual body is precisely analogous to the physical body, so long as the two are conjoined. It is only when they become separated by death that the former is able to live independently.