For, when the friendship is purely spiritual, the love of God grows with it; and the more the soul remembers it, the more it remembers the love of God, and the greater the desire it has for God; so that, as the one grows, the other grows also. For the spirit of God has this property, that it increases good by adding to it more good, inasmuch as there is likeness and conformity between them. But, when this love arises from the vice of sensuality aforementioned, it produces the contrary effects; for the more the one grows, the more the other decreases, and the remembrance of it likewise. If that sensual love grows, it will at once be observed that the soul's love of God is becoming colder, and that it is forgetting Him as it remembers that love; there comes to it, too, a certain remorse of conscience. And, on the other hand, if the love of God grows in the soul, that other love becomes cold and is forgotten; for, as the two are contrary to one another, not only does the one not aid the other, but the one which predominates quenches and confounds the other, and becomes strengthened in itself, as the philosophers say. Wherefore Our Saviour said in the Gospel: 'That which is born of the flesh is flesh, and that which is born of the Spirit is spirit.' That is to say, the love which is born of sensuality ends in sensuality, and that which is of the spirit ends in the spirit of God and causes it to grow. This is the difference that exists between these two kinds of love, whereby we may know them.
As for the vice of lust - aside from what it means for spiritual persons to fall into this vice, since my intent is to treat of the imperfections that have to be purged by means of the dark night - spiritual persons have numerous imperfections, many of which can be called spiritual lust, not because the lust is spiritual but because it proceeds from spiritual things. It happens frequently that in a person's spiritual exercises themselves, without the person being able to avoid it, impure movements will be experienced in the sensory part of the soul, and even sometimes when the spirit is deep in prayer or when receiving the sacraments of Penance or the Eucharist. These impure feelings arise from any of three causes outside one's control. First, they often proceed from the pleasure human nature finds in spiritual exercises. Since both the spiritual and the sensory part of the soul receive gratification from that refreshment, each part experiences delight according to its own nature and properties. The spirit, the superior part of the soul, experiences renewal and satisfaction in God; and the sense, the lower part, feels sensory gratification and delight because it is ignorant of how to get anything else, and hence takes whatever is nearest, which is the impure sensory satisfaction. It may happen that while a soul is with God in deep spiritual prayer, it will conversely passively experience sensual rebellions, movements, and acts in the senses, not without its own great displeasure. This frequently happens at the time of Communion. Since the soul receives joy and gladness in this act of love - for the Lord grants the grace and gives himself for this reason - the sensory part also takes its share, as we said, according to its mode. Since, after all, these two parts form one individual, each one usually shares according to its mode in what the other receives. As the Philosopher says: Whatever is received, is received according to the mode of the receiver. Because in the initial stages of the spiritual life, and even more advanced ones, the sensory part of the soul is imperfect, God's spirit is frequently received in this sensory part with this same imperfection. Once the sensory part is reformed through the purgation of the dark night, it no longer has these infirmities. Then the spiritual part of the soul, rather than the sensory part, receives God's Spirit, and the soul thus receives everything according to the mode of the Spirit.