Sforno on Deuteronomy 9:6/ספורנו על דברים ט׳:ו׳

1כי עם קשה עורף אתה, being stiff-necked and righteous simultaneously is a non starter, as these two character qualities are mutually exclusive. Being stiff-necked means to follow the dictates of one’s own heart, one’s own opinions instead of accepting the yoke of the Creator’s commandments as being supreme. Such a person will reject even theories which have been proven beyond doubt if they conflict with what he deems good or convenient for himself. The simile of a “stiff-necked” person or people described someone who appears unable to turn around and face his teacher, as his neck is too stiff.