Our emotional state is unstable without internal balance. How can heaven last forever? Heaven is everlasting and earth endures the presence. They give each other reason (coexistence) to last forever. Without one another there is no meaning in existence.
Our mind, body and soul functions in coexistence. Our mind functions in coexistence within our toughs. The body functions in coexistence with the organs. Our soul functions in coexistence with our being. From the outer cosmos into the inner cosmos, we can find our nature of heaven and earth. Together we are one and nothing. When an answer can mean everything it loses meaning. This is why we remind us of heaven and earth.
Strengthen the Pillars
In our training we remind us of this principle to overcome unbalance in our system. This way our training does not lose in meaning and we can progress. The more we learn, the more we feel we still have to learn. Sometimes we feel pain, sometimes we feel warm or relaxation, sometimes it feels unbearable. It is because we make ourselves think that way. Without our toughs we would not know. In fact what we feel is life, we should not fear it but embrace it with our heart. Once we start thinking our fear and pain becomes reality. Calming the monkey-mind can overcome ones weakness and strengthen the pillars of heaven and earth.
Believing and Knowing
Believing is for intelligent people who know the truth but never discovered it by themselves. The wise knows the truth, because he understands its origin. In the doctrine of heaven and earth the wise can explain the universe (the way of Tao).