Or HaChaim on Exodus 32:31/אור החיים על שמות ל״ב:ל״א

1 חטא העם הזה, "This nation has committed a very grave sin, etc." It is possible that with these words Moses alluded to something we have learned in Sotah 3, that no one commits a sin unless his brain had first malfunctioned, i.e. that he was out of his mind. This represents a defect in his spiritual life-force, his נפש. By saying חטא, Moses meant that the people's spiritual makeup had lacked an essential ingredient. This was the reason that they made a molten image for themselves. This is one of the arguments man will use after death when he faces His Maker and has to account for his sins in this life. He will claim that when he committed a sin he was not of sound mind. Although he will be punished, seeing he himself was the cause of being of unsound mind, his punishment will not be as severe as if he had been of perfectly sound mind at the time he comitted the sin.

2Another thought Moses may have had in mind when he formulated these words is connected to the tradition (Kidushin 40) that when man is punished for idolatry he is punished for his idolatrous thoughts alone. When Moses spoke of the people having committed a grave sin without specifying the nature of that sin, he referred to the idolatrous thoughts the people had entertained. As far as their sin in deed was concerned, Moses spelled this out by adding: "they have made golden deities for themselves."