Our question now is:
what makes the prophet certain
that he is standing before God?
Seeing with the eyes or in the imagination does not necessarily have anything to do with this. When both are absent there may still be an inner certainty that it is God who is speaking.
This certainty can rest on the feeling that God is present; one feels touched in his innermost being by him, by the One present. We call this the experience of God in the most proper sense. It is the core of all mystical living experience: the person-to-person encounter with God. A sensible vision, like that of Isaiah, may accompany it as an extraordinary attendant phenomenon.
On the other hand,
is a vision like this conceivable
without a personal, inner experience of God?
Saint Edith Stein
Ways to Know God: Experience
4c1) Revelation, Inspiration, and Supernatural Experience of God
Nota Bene: Translator Walter Redmond rendered Erfahrung as experience of God and Erlebnis as mystical living experience.
Knowledge and Faith: The Collected Works of Edith Stein, Vol. 8 ICS Publications, Washington DC © Washington Province of Discalced Carmelites, Inc