Musar zu Wajikra 5:1

מוצא שפתיך תשמור ועשית כאשר נדרת לה' אלוקיך . This verse contains an allusion to the oath of loyalty to the Torah each soul is made to swear by the angel who taught it that same Torah while still in the mother's womb (Niddah 30b). The phrasing "keep your promise," presupposes that everyone has taken such a vow.

We are taught in Sofrim 6,4 that there are three occasions when the Torah writes the word לא whereas we read it as לו. Once in Leviticus 11,21, another time in Leviticus 25,30, and a third time in Leviticus 5,1, where the Torah commenced the verse with the words: ונפש כי תחטא, "When a 'soul' commits an inadvertent transgression, etc." The nature of the transgression is improper use of the mouth, abuse of the power of speech. In this instance the abuse consists of failure to testify, i.e. אם לוא יגיד. The Zohar (Sullam edition page 86) seems to understand the choice of the word נפש as opposed to the word רוח or נשמה, part of our life-force, as indicating that any sin we commit is due only to this inferior part of our "soul." The superior parts such as רוח ונשמה are by themselves quite incapable of committing a sin. The dual spelling of the word לוא is a hint that the רוח ונשמה has to make restitution to the נפש for the purity it has deprived them of by its sin. In other words: What the purely part of man has deprived the רוח, of "לא," has to be restored to the רוח, i.e. לו. Unless this occurs, the result will be: ונשא עונו, "he will bear his sin." This is the first of the three aspects concerning which an individual's lifestyle will be examined after his death, as we mentioned at the beginning of this discussion.