An extract from 'The Ascent of Mount Carmel by St. John of the Cross'
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From this it is evident that, when the soul completely detaches itself, in its will and affection, from the apprehensions of the strains of those forms, images and figures wherein are clothed the spiritual communications which we have described, not only is it not deprived of these communications and the blessings which they cause within it, but it is much better prepared to receive them with greater abundance, clearness, liberty of spirit and simplicity, when all these apprehensions are set on one side, for they are, as it were, curtains and veils covering the spiritual thing that is behind them.
And thus, if the soul desire to feed upon them, they occupy spirit and sense in such a way that the spirit cannot communicate itself simply and freely; for, while they are still occupied with the outer rind, it is clear that the understanding is not free to receive the substance.
Wherefore, if the soul at that time desires to receive these forms and to set store by them, it would be embarrassing itself, and contenting itself with the least important part of them -- namely, all that it can apprehend and know of them, which is the form and image and particular object of the understanding in question. The most important part of them, which is the spiritual part that is infused into the soul, it can neither apprehend nor understand, nor can it even know what it is, or be able to express it, since it is purely spiritual.
All that it can know of them, as we say, according to its manner of understanding, is but the least part of what is in them -- namely, the forms perceptible by sense. For this reason I say that what it cannot understand or imagine is communicated to it by these visions, passively, without any effort of its own to understand and without its even knowing how to make such an effort.