Exodus 7:14

Pharaoh's heart is hardened - כבד cabed , is become heavy or stupid; he receives no conviction, notwithstanding the clearness of the light which shines upon him. We well know the power of prejudice: where persons are determined to think and act after a predetermined plan, arguments, demonstrations, and even miracles themselves, are lost on them, as in the case of Pharaoh here, and that of the obstinate Jews in the days of our Lord and his apostles.