Guarding The Speaking Soul
מוציא שפתיך תשמור הוא שמירת הלשון שהפה הוא אבר פנימי יותר שכל ההבל ופנימיות האדם הוא יוצא ע"י מוצאות הפה ע"כ צריך שמירה יותר
"That which goes out of your lips you shall keep" (Devarim 23:24) refers to the guarding of one's speech, for the mouth is an even more inward organ, since all the vital breath and innermost essence of a person comes forth through the outlets of the mouth; therefore it requires greater guarding.
The Torah's command to keep what leaves your lips is teaching shmiras halashon, because the mouth releases a person's innermost essence and breath, and so it demands extra protection.
ונראה דמצות תלמוד תורה הוא שמירת הפה דכל מ"ע נגד אבר מיוחד ולכן מצוה זו תמיד יום ולילה כי מוצאות הפה צריכין יותר שמירה כנ"ל
And it appears that the mitzvah of talmud Torah is itself the guarding of the mouth, for every positive mitzvah corresponds to a particular organ, and therefore this mitzvah applies constantly, day and night, since the outlets of the mouth require greater guarding, as explained above.
Learning Torah is the very fulfillment of guarding the mouth; since each positive mitzvah is tied to a specific limb, the mitzvah of Torah study applies day and night because the mouth needs constant guarding.
כי כל שורש החיות בהאדם הוא בהבל הפנימי וכשאדם שומר הבל פיו זה שורש לכל המעשים כדכתי' תשמור ועשית כו' שכל המעשים תלוי בשמירת הפה כי יתרון בריאת האדם על הבע"ח כי הוא מדבר
For the entire root of a person's vitality lies in the inner breath, and when a person guards the breath of his mouth, this is the root of all his deeds, as it is written, "you shall keep and you shall do" (Devarim 23:24) - that all deeds depend upon the guarding of the mouth, for the superiority of the creation of man over the animals is that he is a speaking being.
A person's whole life-force is in his inner breath, so guarding his speech becomes the foundation of all his actions, since what elevates man above the animals is his power of speech.
וכ"כ לנפש חי' לרוח ממללא
And so too it is written, "to a living soul" (Bereishis 2:7), which Onkelos renders as "a speaking spirit."
The Torah's phrase "living soul" is translated by Onkelos as "a speaking spirit," underscoring that speech is the defining feature of the human neshamah.
לזאת צריך לתת עיקר המעלה שלו להשי"ת כנ"ל
For this reason a person must give over the essence of his distinction to Hashem Yisbarach, as explained above.
Because speech is man's greatest distinction, he must dedicate that very faculty to Hashem.
כי בריאת הפה ודיבור האדם הוא דבר נפלא יותר מכל הבריאה
For the creation of the mouth and of human speech is something more wondrous than all of creation.
The Sfas Emes stresses that the gift of a mouth and the ability to speak is the most wondrous part of all creation.
כמו שמבין מי שמתבונן בפלא זה
As is understood by one who contemplates this wonder.
Anyone who truly reflects on the marvel of speech will recognize this for himself.
וכל מה שברא הקב"ה הכל לכבודו ברא כמ"ש תהלתי יספרו
And all that the Holy One, Blessed is He, created, He created solely for His honor, as it is written, "My praise shall they recount" (Yeshayahu 43:21).
Everything Hashem created was made for His honor, and the verse "My praise shall they recount" shows that speech in particular exists to express His praise.
ואם כי להוציא דברים לפני השי"ת אינו בנקל אבל עכ"פ שנשמור הפה מדברים בטלים כנ"ל:
And although to bring forth words of Torah and tefillah before Hashem Yisbarach is not an easy matter, nevertheless we should at least guard the mouth from idle words, as explained above.
Even if speaking actual words of Torah and tefillah before Hashem is difficult, at the very least one must guard his mouth from empty, idle talk.
Summary: The Sfas Emes explains that the Torah's command to keep what leaves one's lips is the foundation of shmiras halashon, because the mouth is the most inward of organs, through which a person's vital breath and innermost essence emerge, and therefore demands the greatest guarding. He identifies talmud Torah as the very mitzvah of guarding the mouth, applying day and night because the power of speech is what elevates man above the animals and is the root of all his deeds, as Onkelos renders "living soul" as "a speaking spirit." Since speech is man's greatest distinction and the most wondrous element of all creation, a person must dedicate this faculty back to Hashem Yisbarach, for all that the Holy One, Blessed is He, created was made for His honor. The practical avodah is that even though filling one's mouth with words of Torah and tefillah is no easy matter, one must at the very least guard the mouth from idle, empty words.