"22 The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light.
23 But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is that darkness!" ~Matt 6 KJV
It says "eye", not "eyes". I think it's talking about the third eye, the spiritual eye. Modern medicine calls it the Pineal gland. It's the very centermost structure in the brain, and is the seat of our imagination. Containing rods & cones, the same structure that detect contrast & color in the retina, it is where we visualize things, where we "see" what we read in books. The cross section of the Pineal gland is identical in structure to the common stylized "eye" included in many Egyptian hieroglyphs.
That eye one truly does fill with light or darkness, depending on what we fill our minds with, what our minds dwell on, and what we receive into it and create with it.
Father, I love Your Word. I love filling my mind with scripture, meditating on it, writing about it, praying over it. And I love most of all what happens when I do those things...the singleness of mind that comes more and more often, and the radical jump I feel in my connection to You. I hear Your Spirit speak so much more clearly. I see what You want to reveal so much more easily. I feel the subtle nudges and checks in the Holy Spirit so much more easily. Thank You. And thank You for this incredible blessing of scripture that is such a powerful vehicle for drawing closer to You. In Jesus' name, I pray. Amen.