"This picture establishes the direct relationship between vision and knowledge for which the Dominican Aquinas had argued in his Summa Theologica. Just as we still  use the phrase "I see" to mean "I understand," For Aquinas the word visio meant more than just vision. "This term," he writes, "in view of the special nature and certitude of sight, is extended in common usage to the knowledge of all the senses and it is even made to include intellectual knowledge, as in Matthew 5:8: ‘Blessed are the pure in heart, for they shall see God.’" The Pisa altarpiece, like most Gothic images, was not considered primarily as a work of art by its contemporaries, but as something far more powerful and instrumental, because of its capacity not just to reflect the world, but to reshape it in God’s image."

-Michael Camille, Gothic Art – Glorious Visions, 25.