O dearest daughter, open wide your mind's eye and look into the abyss of my charity. There is not a person whose heart would not melt in love to see, among all the other blessings I have given you, the blessing you receive in this sacrament.

And how, dearest daughter, should you and others look upon this mystery and touch it? Not only with your bodily eyes and feeling, for here they would fail you. You know that all your eyes see is this white bit of bread; this is all your hand can touch and all your tongue can taste, so that your dull bodily senses are deceived. But the soul's sensitivity cannot be deceived, unless she so chooses by extinguishing the light of holy faith by infidelity.

What tastes and sees and touches this sacrament? The soul's sensitivity. How does she see it? With her mind's eye, so long as it has the pupil of holy faith. This eye sees in that whiteness the divine nature joined with the human; wholly God, wholly human; the body, soul, and blood of Christ, his soul united with his body and his body and soul united with my divine nature, never straying from me. This, if you remember, is what I revealed to you early in your life and that not only to your mind's eye but to your bodily eyes as well, although because of the great light you soon lost your bodily sight and were left with only your spiritual vision.

The Dialogue
St. Catherine of Siena