וַיֹּ֕אמֶר לֹ֥א תוּכַ֖ל לִרְאֹ֣ת אֶת־פָּנָ֑י כִּ֛י לֹֽא־יִרְאַ֥נִי הָאָדָ֖ם וָחָֽי׃
און ער האָט געזאָגט: קענסט ניט זען מײַן פּנים, װאָרום אַ מענטש קען מיך ניט זען און בלײַבן לעבן.
Rashi
ויאמר לא תוכל וגו׳ AND HE SAID, THOU CANST NOT [SEE MY FACE] — and even when I make all My goodness pass before you I shall not allow you to see My face.
Chizkuni
כי לא יראני האדם וחי, “for as long as a human being is alive, he cannot experience My essence visually.” According to Rabbi Shimon the Yemenite,” the definition of a human being is that he is alive. But even celestial creatures named חיות are named thus to remind us that though they “live” forever, they too are not able to have a visual experience of G-d’s essence (Torah Shleymah 131 on our verse.) If you were to counter that Isaiah 6,2 claimed to have experienced such a visual revelation when he said: ואראה את ה׳אדוני יושב על כסא, “I have seen G-d seated on a throne,” this did not describe what is known as a clear vision, but as something at best like a reflection from a mirror; he had been screened by a partition at the time.
Ramban
FOR MAN SHALL NOT SEE ME AND LIVE. This does not mean that man could see Him, but must immediately die; it means that before he could grasp the sight, his soul would depart from him, for even of the vision of the angels it is said, by reason of the vision my pains are come upon me, [and I retained no strength].496Daniel 10:16.