Rabbi Roodyn

שָׁמוֹר וְזָכוֹר בְּדִבּוּר אֶחָד. הִשְׁמִיעָנוּ אֵל הַמְיֻחָד. ה’ אֶחָד וּשְׁמוֹ אֶחָד. לְשֵׁם וּלְתִפְאֶרֶת וְלִתְהִלָּה:

‘Safeguard!’ And ‘Remember’ in a single utterance  the One and Only G-d made us hear. Hashem is One and His Name is One, for renown, splendour and praise.

As is well known, the two accounts of  the ten commandments have a number of variations.  In Yisro we are told to remember the Sabbath day, whereas in vaeschanan we are told to safeguard it. Chazal tell us that when then Ten Commandments were given both words were uttered at once, a feat that no human being is capable of performing.

Zachor is the idea that we are meant to remember Shabbos in our thoughts and to verbalise our expressions of the importance of Shabbos as an eternal sign of Hashem’s creation of the universe. Shamor is the physical demonstration of that idea through desisting from melacha.

Rav Hirsch in his commentary to the siddur explains the depth behind this idea. He says that the fact that they were uttered together shows that both of these concepts are inseparable. If we would only have zachor, a mere remembrance, we would be missing the entire message of the word Shabbos, which means to desist (from melacha). This theoretical observance would be a denial of our ability to pay homage to Hashem through our actions, as if they were insignificant. Similarly Shamor without Zachor misses the point too. Merely desisting from work without accepting its significance in spirit would turn Shabbos into a day of rest devoid of any deeper significance.