The Alter Rebbe explains that creation is not a one-time past event but a continuous divine speech-act: God's word constantly animates and sustains every creature, and without it, each thing would revert to absolute nothingness (ayin gamur).
וְהִנֵּה, אַחֲרֵי הַדְּבָרִים וְהָאֱמֶת הָאֵלֶּה, כָּל מַשְׂכִּיל עַל דָּבָר יָבִין לַאֲשׁוּרוֹ, אֵיךְ שֶׁכָּל נִבְרָא וָיֵשׁ הוּא בֶּאֱמֶת נֶחְשָׁב לְאַיִן וָאֶפֶס מַמָּשׁ לְגַבֵּי כֹּחַ הַפּוֹעֵל וְרוּחַ פִּיו שֶׁבַּנִּפְעָל, הַמְהַוֶּה אוֹתוֹ תָּמִיד וּמוֹצִיאוֹ מֵאַיִן מַמָּשׁ לְיֵשׁ. וּמַה שֶּׁכָּל נִבְרָא וְנִפְעָל נִרְאֶה לָנוּ לְיֵשׁ וּמַמָּשׁוּת, זֶהוּ, מֵחֲמַת שֶׁאֵין אָנוּ מַשִּׂיגִים וְרוֹאִים בְּעֵינֵי בָשָׂר אֶת כֹּחַ ה׳ וְרוּחַ פִּיו שֶׁבַּנִּבְרָא, אֲבָל אִילּוּ נִיתְּנָה רְשׁוּת לָעַיִן לִרְאוֹת וּלְהַשִּׂיג אֶת הַחַיּוּת וְרוּחָנִיּוּת שֶׁבְּכָל נִבְרָא, הַשּׁוֹפֵעַ בּוֹ מִ״מּוֹצָא פִי ה׳״ וְרוּחַ פִּיו – לֹא הָיָה גַּשְׁמִיּוּת הַנִּבְרָא וְחוּמְרוֹ וּמַמָּשׁוֹ נִרְאֶה כְּלָל לְעֵינֵינוּ, כִּי הוּא בָּטֵל בִּמְצִיאוּת מַמָּשׁ לְגַבֵּי הַחַיּוּת וְהָרוּחָנִיּוּת שֶׁבּוֹ, מֵאַחַר שֶׁמִּבַּלְעֲדֵי הָרוּחָנִיּוּת, הָיָה אַיִן וָאֶפֶס מַמָּשׁ, כְּמוֹ קוֹדֶם שֵׁשֶׁת יְמֵי בְרֵאשִׁית מַמָּשׁ, וְהָרוּחָנִיּוּת הַשּׁוֹפֵעַ עָלָיו מִ״מּוֹצָא פִי ה׳״ וְרוּחַ פִּיו, הוּא לְבַדּוֹ הַמּוֹצִיאוֹ תָּמִיד מֵאֶפֶס וָאַיִן לְיֵשׁ, וּמְהַוֶּה אוֹתוֹ, אִם כֵּן, אֶפֶס בִּלְעָדוֹ בֶּאֱמֶת.
Now, following these words and the truth [concerning the nature of the Creation], every intelligent person will understand clearly that each creature and being is actually considered naught and absolute nothingness in relation to the Activating Force and the “breath of His mouth” which is in the created thing, continuously calling it into existence and bringing it from absolute nonbeing into being. The reason that all things created and activated appear to us as existing and tangible is that we do not comprehend nor see with our physical eyes the power of G–d and the “breath of His mouth” which is in the created thing. If, however, the eye were permitted to see and to comprehend the life-force and spirituality which is in every created thing, flowing into it from “that which proceeds out of the mouth of G–d” and “His breath,” then the materiality, grossness, and tangibility of the creature would not be seen by our eyes at all, for it is completely nullified in relation to the life-force and the spirituality which is within it, since without the spirituality it would be naught and absolute nothingness, exactly as before the Six Days of Creation. The spirituality which flows into it from “that which proceeds out of the mouth of G–d” and “His breath”—that alone continuously brings it forth from naught and nullity into being and gives it existence. Hence, there is truly nothing besides Him.