We are led to Believe a Lie When we see not Thro' the Eye Which was Born in a Night to Perish in a Night When the Soul Slept in Beams of Light. (125-128)
If you're seeing "through" your eye, you're approaching your eye as a tool you have. You're looking through it the same way you'd look through a telescope or a microscope. What you're really looking with is the spirit—which is behind the eye and uses it as this tool. That way, you can see hidden poetic dimensions of reality—the "world in a grain of sand" from the very beginning.
But, if you just see with your eye, then you're being deluded. Since Blake views the physical world as an illusion—a distorted image of the spiritual world—he thinks that you can't just take everything at face value. Real imagination and spiritual power are needed to perceive the eternal realities that genuinely exist—like reading a Where's Waldo book, except with God instead of Waldo or something.