Mark 6:52

For they understood not

(ου γαρ συνηκαν). 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 (καρδια in the general sense for all the inner man)was hardened

(ην πεπωρωμενη). See on Mark 3:5 about πωρωσις. 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.