 He uses a tree as the kind of metaphor within this parable. He points out that a tree will only be fruitful if it's rooted in a soil that truly nourishes it. We know that if we were to uproot a tree from soil, it would not be healthy and eventually would bear no fruit. Just so in the spiritual life, the roots of the tree of the soul have to be sunk deep into the soil of humility. So the roots of the soul, the roots of the tree of the soul have to go deep into humility. Humility is the foundation. Why? Because it produces true knowledge of the self and of God. When we know that God is everything, when we know that apart from Him we can do nothing, when we know ourselves as utterly dependent on Him, we are rooted in that rich soil of humility.