It is the heart that God wants us to understand. By understanding the heart, we can understand why we suffer or have joy. As you continue to read the verses, ponder how it is related to the heart, that God wants us to understand.
God is telling us to ponder our own hearts, and calm our minds when illness as guidance is given. God wants us to understand the heart, in order to purify our minds by ourselves. This is the path of the heart, which will bring upon the Joyous life.