After the sin of golden-calf, God terms us a “stiff-necked people.” The Hebrew word used is “oref,” which really means the back and bottom of the head. The Netziv quotes a medieval view that this part of the brain was responsible for forming habits and making decisions – God was blaming our neurology! Yet, later Moses instructs us to circumcise the foreskin of your heart, and your neck will not be stiff anymore. The message of the Torah is that we can, and sometimes must, overcome our natural disposition.