For they understood not (ou gar sunhkan). Explanation of their excessive amazement, viz., their failure to grasp the full significance of the miracle of the loaves and fishes, a nature miracle. Here was another, Jesus walking on the water. Their reasoning process (kardia in the general sense for all the inner man) was hardened (hn pepwrwmenh). See on 3:5| about pwrwsi. Today some men have such intellectual hardness or denseness that they cannot believe that God can or would work miracles, least of all nature miracles.