Since the time of Adam, Christ, the Wisdom of the Father, has said to all men, and He says so still, inwardly according to His Divinity: Behold. And this beholding is needful. Now mark this well: that for anyone who wishes to see, either in a bodily or a ghostly manner, three things are necessary.
The first thing is that, if a man will see bodily and outwardly, he must have the outward light of heaven, or some other material light, to illuminate the medium, that is, the air, through which he will see. The second thing is, that he must permit the things which he wishes to see to be reflected in his eyes. And the third thing is that the organs, the eyes, must be sound and flawless, so that gross bodily things can be subtly reflected in them. If a man lack any of these three things his bodily sight is wanting. Of this sight, however, we shall say nothing more; but we shall speak of a ghostly and supernatural sight, in which all our bliss abides.
For all who wish to see in a ghostly and supernatural manner three things also are needful. The first is the light of Divine grace; the second is a free turning of the will to God, the third is a conscience clean from any mortal sin.
St. John Ruysbroeck, Adornment of the Spiritual Marriage