All evil spirits constitute the third enemy of man's salvation. Moved by hatred and envy they do all in their power to bring man to perdition. Not content with using the influence of the world to turn him from the path of virtue, they exert their influence on him personally to attain their end. Though they cannot influence man's mind and heart directly, they can inflict great harm on them through his senses and his passions.
The devil may act on man's external senses, (1) by an illusory sensation and make a corresponding impression on the imagination and memory; (2) by a corporeal apparition, as he appeared to the Saviour in the desert. He may act directly on man's internal senses (1) by inciting the instinct to carnal desires; (2) by effacing virtuous impressions from the imagination and memory; (3) and impressing vicious images in their stead ; (4) by fixing such vicious impressions deeply upon the internal senses. In this way the devil distracts the mind of a child of God, harasses his will, and inclines his heart to sin, counteracts the past effects of grace and virtue, blinds man to the blessings of the present, and tempts him to sin.
Ordinarily the devil is too shrewd to tempt man directly to embrace error or to act maliciously. In fact he accomplishes his end more securely by using the evil impressions of the imagination and memory and the bad example and friendship of the world to instill perverse principles in the human mind and selfish motives in the human heart, and thereby puts man at variance with God. In this way the devil does not shock man, but easily leads him to hold, that, as his influence cannot be easily detected, it has been much overrated, especially in modern times.