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.

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. –St John of the Cross ‘Dark Night of the Soul’.