the mind > emotion > absence of emotion > lack sensitivity [verb (intransitive)] > be callous or hard-hearted > become callous or hard-hearted (3)
intransitive. To become numbed.
intransitive. To become callous or unfeeling; to act more harshly or severely.
figurative. intransitive. To become emotionally hardened or callous; to become rigid or fixed in attitude, etc.