John of the Cross,
Ascent of Mount Carmel
“ It is impossible to explain in words, or to cause to be understood by the understanding, what variety of impurity is caused in the soul by a variety of desires. For, if it could be expressed and understood, it would be a wondrous thing, and one also which would fill us with pity, to see how each desire, in accordance with its quality and degree, be it greater or smaller, leaves in the soul its mark and deposit of impurity and vileness, and how one single disorder of the reason can be the source of innumerable different impurities, some greater, some less, each one after its kind. ”
