21 But if it be goaded on and inflamed with deceitful visions and unclean incentives by the devilish spirit, associated and conspiring therewith in malignant agreement, this spirit makes the will of the man either frantic with error, or burning with appetite of some worldly delight; and hence, it seems to show a marvellous endurance of intolerable evils: but yet it does not follow from this that an evil will without instigation of another and unclean spirit, like as a good will without aid of the Holy Spirit, cannot exist.
For that there may be an evil will even without any spirit either seducing or inciting, is sufficiently clear in the instance of the devil himself, who is found to have become a devil, not through some other devil, but of his own proper will. An evil will therefore, whether it be hurried on by lust, whether called back by fear, whether expanded by gladness, whether contracted by sadness, and in all these perturbations of mind enduring and making light of whatever are to others, or at another time, more grievous, this evil will may, without another spirit to goad it on, seduce itself, and in lapsing by defection from the higher to the lower, the more pleasant it shall account that thing to be which it seeks to get or fears to lose, or rejoices to have gotten, or grieves to have lost, the more tolerably for its sake bear what is less for it to suffer than that is to be enjoyed.
For whatever that thing be, it is of the creature, of which one knows the pleasure. Because in some sort, the creature loved approaches itself to the creature loving in fond contact and connection, to the giving experience of its sweetness.
Source: On Patience (New Advent)