From stanza 11 of The Spiritual Canticle.

Moses, on Mount Sinai in the presence of God, saw such glimpses of the majesty and beauty of His hidden Divinity, that, unable to endure it, he prayed twice for the vision of His glory, saying: “Whereas You have said: I know you by name, and you have found grace in my sight. If, therefore, I have found grace in Your sight, show me Your face, that I may know You and may find grace before Your eyes”; that is, the grace which he longed for — to attain to the perfect love of the glory of God.

The answer of our Lord was: “You can not see My face, for man shall not see Me and live.”

It is as if God had said: “Moses, your prayer is difficult to grant; the beauty of My face, and the joy in seeing Me is so great, as to be more than your soul can bear in a mortal body that is so weak.”

The soul accordingly, conscious of this truth, either because of the answer made to Moses or also because of that which I spoke of before, namely the feeling that there is something still in the presence of God here which it could not see in its beauty in the life it is now living, because, as I said before, it faints when it sees but a glimpse of it.