The soul that is clouded by the desires is darkened in the understanding, and allows neither the sun of natural reason nor that of the supernatural Wisdom of God to shine upon it and illumine it clearly.
[…] And, at this same time, when the soul is darkened in the understanding, it is benumbed also in the will, and the memory becomes dull and disordered in its due operation.
For, as these faculties in their operations depend upon the understanding, it is clear that, when the understanding is impeded, they will become disordered and troubled.
And thus David says: “My soul is sorely troubled”. Which is as much as to say, “disordered in its faculties.”
For, as we say, the understanding has no more capacity for receiving enlightenment from the wisdom of God than has the air, when it is dark, for receiving enlightenment from the sun.
Neither has the will any power to embrace God within itself in pure love, even as the mirror that is clouded with vapour has no power to reflect clearly within itself any visage.
And even less power has the memory, which is clouded by the darkness of desire, to take clearly upon itself the form of the image of God, just as the muddled water cannot show forth clearly the visage of one that looks at himself therein.
Desire blinds and darkens the soul; for desire, as such, is blind, since of itself it has no understanding in itself, the reason being to it always, as it were, a child leading a blind man.
And hence it comes to pass that, whensoever the soul is guided by its desire, it becomes blind; for this is as if one that sees were guided by one that sees not, which is, as it were, for both to be blind.
[…] And even so we may say that one who feeds upon desire is like a fish that is dazzled, upon which the light acts rather as darkness, preventing it from seeing the snares which the fishermen are preparing for it.
[…] And it is this that desire does to the soul, enkindling its concupiscence and dazzling its understanding so that it cannot see its light.
For the cause of its being thus dazzled is that when another light of a different kind is set before the eye, the visual faculty is attracted by that which is interposed so that it sees not the other.
And, as the desire is set so near to the soul as to be within the soul itself, the soul meets this first light and is attracted by it.
And thus it is unable to see the light of clear understanding, neither will see it until the dazzling power of desire is taken away from it.
John of the Cross (1542-1591): Ascent of Mount Carmel, 1, 8, 1-3.