From The Dark Night of the Soul of San Juan de la Cruz:
2. ...in order that the soul may be divinely prepared and tempered with its faculties for the Divine union of love, it would be well for it to be first of all absorbed, with all its faculties, in this Divine and dark spiritual light of contemplation, and thus to be withdrawn from all the affections and apprehensions of the creatures, which condition ordinarily continues in proportion to its intensity. And thus, the simpler and the purer is this Divine light in its assault upon the soul, the more does it darken it, void it and annihilate it according to its particular apprehensions and affections, with regard both to things above and to things below; and similarly, the less simple and pure is it in this assault, the less deprivation it causes it and the less dark is it. Now this is a thing that seems incredible, to say that, the brighter and purer is supernatural and Divine light, the more it darkens the soul, and that, the less bright and pure is it, the less dark it is to the soul. Yet this may readily be understood if we consider what has been proved above by the dictum of the philosopher-namely, that the brighter and the more manifest in themselves are supernatural things the darker are they to our understanding.
3. And, to the end that this may be understood the more clearly, we shall here set down a similitude referring to common and natural light. We observe that a ray of sunlight which enters through the window is the less clearly visible according as it is the purer and freer from specks, and the more of such specks and motes there are in the air, the brighter is the light to the eye. The reason is that it is not the light itself that is seen; the light is but the means whereby the other things that it strikes are seen, and then it is also seen itself, through its reflection in them; were it not for this, neither it nor they would have been seen. Thus if the ray of sunlight entered through the window of one room and passed out through another on the other side, traversing the room, and if it met nothing on the way, or if there were no specks in the air for it to strike, the room would have no more light than before, neither would the ray of light be visible. In fact, if we consider it carefully, there is more darkness where the ray is, since it absorbs and obscures any other light, and yet it is itself invisible, because, as we have said, there are no visible objects which it can strike.
4. Now this is precisely what this Divine ray of contemplation does in the soul. Assailing it with its Divine light, it transcends the natural power of the soul, and herein it darkens it and deprives it of all natural affections and apprehensions which it apprehended aforetime by means of natural light; and thus it leaves it not only dark, but likewise empty, according to its faculties and desires, both spiritual and natural. And, by thus leaving it empty and in darkness, it purges and illumines it with Divine spiritual light, although the soul thinks not that it has this light, but believes itself to be in darkness, even as we have said of the ray of light, which although it be in the midst of the room, yet, if it be pure and meet nothing on its path, is not visible. With regard, however, to this spiritual light by which the soul is assailed, when it has something to strike-that is, when something spiritual presents itself to be understood, however small a speck it be and whether of perfection or imperfection, or whether it be a judgment of the falsehood or the truth of a thing-it then sees and understands much more clearly than before it was in these dark places. And exactly in the same way it discerns the spiritual light which it has in order that it may readily discern the imperfection which is presented to it; even as, when the ray of which we have spoken, within the room, is dark and not itself visible, if one introduce a hand or any other thing into its path, the hand is then seen and it is realized that that sunlight is present.
5. Wherefore, since this spiritual light is so simple, pure and general, not appropriated or restricted to any particular thing that can be understood, whether natural or Divine (since with respect to all these apprehensions the faculties of the soul are empty and annihilated), it follows that with great comprehensiveness and readiness the soul discerns and penetrates whatsoever thing presents itself to it, whether it come from above or from below; for which cause the Apostle said: That the spiritual man searches all things, even the deep things of God. For by this general and simple wisdom is understood that which the Holy Spirit says through the Wise Man, namely: That it reaches wheresoever it wills by reason of its purity; that is to say, because it is not restricted to any particular object of the intellect or affection. And this is the characteristic of the spirit that is purged and annihilated with respect to all particular affections and objects of the understanding, that in this state wherein it has pleasure in nothing and understands nothing in particular, but dwells in its emptiness, darkness and obscurity, it is fully prepared to embrace everything...