See his interactive updated story here. Our own life is the instrument with which we experiment with truth. A man is born gentle and weak. At his death he is hard and stiff.
Green plants are tender and filled with sap.
At their death they are withered and dry. Therefore the stiff and unbending is the disciple of death. The gentle and yielding is the disciple of life. Thus an army without flexibility never wins a battle. A tree that is unbending is easily broken.