Shabbos guards all the mitzvos

בפסוק ששת ימים כו' השביעי תשבות כו' ובכל אשר אמרתי אליכם תשמרו כו' סמך הקב"ה זה למצות שבת לומר כי שקול שבת ככל המצות.

On the pasuk "six days… and on the seventh you shall rest… and in all that I have said to you, you shall be careful…" — Hashem juxtaposed this to the mitzvah of Shabbos, to say that Shabbos is equal to all the mitzvos.

The Sefas Emes notes that the Torah places "guard all that I have said to you" right next to Shabbos, teaching that Shabbos is weighed against all the mitzvos combined.

וכבר כתבנו פירוש למען ינוח שורך כו' הוא המשכת הארת מנוחת שבת גם במעשה גופניות.

And we have already written an explanation of "so that your ox may rest…" — it is the drawing down of the illumination of the rest of Shabbos even into physical activity.

The mitzvah that even one's animal rest on Shabbos means that the light of Shabbos menuchah is extended downward even into the physical, bodily realm.

שהוא בחי' נפש הבהמיות.

Which is the aspect of the animal soul (nefesh habahamis).

The "ox" represents the nefesh habahamis — the animalistic life-force within a person, the bodily drives.

לכן שם הקב"ה ששת ימי המעשה ושבת.

Therefore Hashem established the six workdays and Shabbos.

Hashem set up the rhythm of six days of activity and the seventh of rest precisely so that the Shabbos light could reach back and elevate even the weekday physical labor.

והנה רש"י פי' ובכל כו' תשמרו.

Now, Rashi explained "and in all… you shall be careful."

The Sefas Emes turns to Rashi's reading of the clause "you shall guard."

לעשות כל התורה מצות ל"ת ע"ש.

To make all of the Torah's [negative] commandments into mitzvos lo sa'aseh (prohibitions) — see there.

Rashi explains that "tishmoru" (you shall guard) attaches the language of "guarding" to every prohibition in the Torah, so that each one carries the force of a negative command.

והענין מובן כי כל המצות הם עצות ושמירה לכל הרמ"ח איברים.

And the matter is understood, for all the mitzvos are counsels and safeguards for all the 248 limbs.

The mitzvos function as etzos (counsels) and protections for the 248 limbs of a person — each mitzvah guards and gives life to its corresponding limb.

ומכלל דכתיב תשמרו צריך האיש לשמירה שלא יתגבר עליו נפש הבהמיות כי יש תערובות טו"ר באדם.

And from the fact that it is written "you shall guard," we learn that a person needs safeguarding, lest the animal soul overpower him — for there is an admixture of good and evil within man.

The very language of "guarding" implies a danger: because man contains a mixture of tov and ra, he must protect himself so that the nefesh habahamis does not gain the upper hand.

וז"ש תשמרו ושם אלהים אחרים לא תזכירו כו'.

And this is the meaning of "you shall guard, and the name of other gods you shall not mention…"

The pasuk continues from "you shall guard" directly to "do not mention other gods," teaching that the goal of all this safeguarding is to keep a person from any pull toward avodah zarah.

שזה תכלית שמירה של המצות.

For this is the ultimate purpose of the safeguarding of the mitzvos.

The whole protective power of the mitzvos aims at this end: keeping a person wholly bound to Hashem and free of any foreign attachment.

ונמצא כשמחסר מצות עשה נחסר שמירה לאבר ההוא וגובר בו הפסולת.

It emerges that when one omits a positive mitzvah, the safeguard for that limb is lacking, and the dross gains strength within it.

Failing to perform a mitzvas asei leaves its corresponding limb unguarded, allowing the "pesoles" — the coarse, evil element — to overpower that part of the person.

והרי זה כמצות לא תעשה.

And this is like a negative commandment.

Thus neglecting a positive mitzvah effectively functions like transgressing a prohibition, since it removes the guard and lets evil take hold — which is precisely Rashi's point that "tishmoru" turns even omissions into matters of "guarding."

ושבת השקול ככל המצות כתיב בו שמירה.

And Shabbos, which is equal to all the mitzvos, has "guarding" (shemirah) written in connection with it.

Fittingly, the Torah uses the language of "shamor" (guard) for Shabbos — for Shabbos, being equal to all the mitzvos, encompasses the protective power of every one of them.

וזה למען ינוח שורך הכנעת נפש הבהמיות כנ"ל.

And this is "so that your ox may rest" — the subjugation of the animal soul, as above.

The "rest of the ox" means the quieting and subduing of the nefesh habahamis through the Shabbos light, the very safeguard the mitzvos provide.

וממילא מאיר נפש האדם.

And automatically the soul of man shines forth.

Once the animal soul is subdued, the higher nefesh ha'adam — the human, holy soul — naturally shines and is revealed.

וזה כל שישנו בשמירה ישנו בזכירה:

And this is "whoever is included in 'guarding' (shamor) is included in 'remembering' (zachor)."

Chazal's rule linking shamor and zachor of Shabbos now has a deeper meaning: when one fulfills the "guarding" — subduing the animal soul and protecting every limb — one thereby attains the "remembering," the positive radiance of the soul's connection to Hashem.

Summary: Shabbos is equal to all the mitzvos and carries the language of "guarding," because all mitzvos are safeguards for the 248 limbs, protecting a person from the animal soul that would otherwise let the "dross" prevail. Even omitting a positive mitzvah is like transgressing a prohibition, since it leaves a limb unguarded. Shabbos draws the light of menuchah even into the physical, subduing the nefesh habahamis so that the holy soul shines — and thus "whoever is in shamor is in zachor."