“When your eyes are healthy, your whole body also is full of light. But when they are unhealthy, your body also is full of darkness. See to it, then, that the light within you is not darkness.”
Your perspective of things plays an important role in your life. Thus, the verse says that if your eyes are filled with darkness, so your whole being will be. If your eye is filled with good things of which God is pleased, it will guide you to the right direction and will make you hopeful even when being tested.