Jesus compares spiritual growth to a seed buried in the earth. He concludes, ""he knows not how,"" meaning growth and fruit-bearing are a mystery. It's unseen and uncomprehended by mortal humanity. Spiritual growth is by God's own sovereign will. Yet, we can stunt it by neglect, as a slothful farmer neglects his plants (Hebrews 2:3). We can severely damage it by corrupting it. Spiritually, we can't mix grace and works, the gospel and the world.