Musar sur Les Nombres 30:4

In פרשת מטות the Torah deals with laws designed to perfect our souls, our spiritual development. The laws dealing with vows contain the warning to honor all our utterances, ככל היוצא מפיו יעשה "he must carry out all that crosses his lips" (Numbers 30,4). This is addressed to the soul, because the power of speech derives from the attribute of wisdom which itself is an outgrowth of the soul. Speech is only an externalized version of one's thoughts, something that elevates man above all other living creatures. Man is called a מדבר, "a talking creature." This is why he must not "profane" his word, "לא יחל דברו." We also find that Onkelos renders Genesis 2,7: ויהי האדם לנפש חיה as "man became a talking spirit." You are already aware that man has been designed along the superior lines of a higher world, that each one of his limbs is a branch of a "tree" in the Celestial Regions, and that the mouth he has been equipped with is meant only to enable him to proclaim the greatness of the Lord and to tell His praises. It is a vessel to be used only for the service of the Lord. This is what David meant when he said in Psalms 145,21: "My mouth shall utter the praise of the Lord."