Epictetus, Golden Sayings 23

Petrifaction is of two sorts. There is petrifaction of the understanding, and also of the sense of shame. 

This happens when a man obstinately refuses to acknowledge plain truths, and persists in maintaining what is self-contradictory. 

Most of us dread mortification of the body, and would spare no pains to escape anything of that kind. 

But of mortification of the soul we are utterly heedless. With regard, indeed, to the soul, if a man is in such a state as to be incapable of following or understanding anything, I grant you we do think of him in a bad way. 

But mortification of the sense of shame and modesty we go so far as to call strength of mind!