Jan van Ruusbroec (1293—1381), was a Flemish mystic and one of the most original thinkers in the medieval West. It's a short fragment from his book "The Spiritual Espousals" in translation of Wim van den Dungen.
Now, if the spirit is to contemplate God with God, without intermediary, in this divine light, three things are necessary for a person.
The first is that he must be well-ordered from without in all virtues and unhindered within, just as empty of all outward works as though he were not working. For if he is busy within by any work of virtue, then he is assailed by images. As long as that is going on in him, he cannot contemplate.
Secondly, he must cleave to God within by devoted intention and love, just like a kindled, blazing fire that can no longer be extinguished. During the time that he feels himself to be in this state, he can contemplate.
Thirdly, he must have lost himself in a waylessness and in a darkness in which all contemplatives wander around in enjoyment and can no longer find themselves in a creaturely way. In the abyss of this darkness in which the loving spirit has died to itself, there begin the revelation of God and eternal life. For in this darkness there shines and is born an incomprehensible light which is the Son of God, in whom one contemplates eternal life.
And in this light one becomes seeing.
And this divine light is granted in the simple being of the spirit, where the spirit receives the brightness –which is God Himself- above all gifts and above all creaturely activity, in the empty void of the spirit in which it has lost itself through enjoyable love and receives the brightness of God without intermediary. And without cease, it becomes the very brightness which it receives.
See, this hidden brightness in which one contemplates everything that one desires according to the way of emptiness of spirit, this brightness is so great that the loving contemplative neither sees nor feels anything in his ground in which he rests except an incomprehensible light. And according to the simple bareness which encompasses all things, he finds and feels himself to be that very light by which he sees, and nothing else.
And here You have the first point : how one becomes seeing in Divine Light. Blessed are the eyes which are thus seeing, for they posses eternal life.