Vision is critical to us both spiritually and physically.  Due to the retina's vital role in vision, damage to it can cause permanent blindness. The Word of God is our spiritual retina.  Conditions such as retinal detachment, where the retina is abnormally detached from its usual position, can prevent the retina from receiving or processing light. This prevents the brain from receiving this information, thus leading to blindness.  In the same way, as we studied the agenda of linking and unlinking (or fastening/unfastening) we can become spiritually detached thus causing a spiritual blindness and inability to see what God would have for us. Isaiah says, "He has blinded their eyes and hardened their heart, so that they might not perceive with their eyes, and understand with their mind and turn, and I would heal them."