Psalms 24:4

who has not taken My name in vain Who has not sworn with My name and My soul in vain. We find an expression of an oath used with the soul (נפש), as it is stated (in Amos 6:8): “The Lord God swore by Himself (בנפשו).”

The clean of hand and pure of heart, who hath not lifted up his soul unto vanity, And hath not sworn deceitfully: – Notice that he indicates in this verse three dispositions in man, action, thought, and speech; and in the three of them the man will be perfect who shall be clean in his actions, pure in his heart, and trustworthy in his speech. (The marginal reading) is נפשׁי (written) with yodh (= my soul), and is the utterance of God, as (in the verses) (Exod. 20:7; Deut. 5:11), “Thou shalt not take the name of the Lord thy God in vain”; and נפשׁו (His soul, essence, being) is (equivalent to) His Name. And so it says (in the verse) (Amos. 6:8), “The Lord God hath sworn by Himself (בנפשׁו).” And the word as written, נפשׁו, i.e. with waw, means his soul, viz. the swearer's; a man shall not lift up unto vanity nor swear deceitfully, but in truth and righteousness. And such swearing is a mitzvah to the fearers of the Name, as is said (in the text) (Deut. 6:13), “And shalt swear by His Name”; and another verse says (ibid. 10:20), “The Lord thy God shalt thou fear, Him shalt thou serve, and to Him shalt thou cleave, and by His Name shalt thou swear.” When these attributes (i.e. service, fear, adhesion) are thine, then by His Name shalt thou swear. And so it says (Jer. 4:2), “And thou shalt swear ‘As the Lord liveth,’ in truth, in judgment, and in righteousness.”