They must not fear, nor regard as sin, or take to heart any evil impulses to sin or to blasphemy, or doubts about the Sacrament, or any other such ugly temptations; for to experience these temptations defiles the soul no more than the bark of a dog or the bite of a flea. They trouble the soul but do not harm it provided a man puts them aside and ignores them. It does no good to struggle against them, or to try and master them by force, for the more a person struggles against them, the more persistent they become.

Book I, ch. 38 (p. 43) - The Ladder of Perfection (1494)