20. PREFACE - Every bad thought can have two origins: our own spiritual imperfection or the action of a harmful influence. In the last case we have the indication of a weakness which exposes us to these influences, and it is for this reason that our soul is imperfect. So that the one who foils cannot offer as an excuse the influence of a strange Spirit, seeing that this Spirit would not hove led them to wrongdoing if they were inaccessible to seduction.
When we have a bad thought, we can suppose that it was an evil Spirit which suggested the evil, leaving us complete liberty to accede or resist, just as if we were facing a living person. At the same time, we should make a mental picture of our Guardian Angel or protecting Spirit who from his side combats within us the bad influences and anxiously awaits the decision we are going to make. Our hesitation in acting upon the evil suggestions is due to the voice of our good Spirit, who makes himself heard through our conscience.
One recognises a thought is bad when it draws away from charity which is the base of all true morality; or when it comes laden with pride, vanity and selfishness; or when its realization may cause harm to another person: in short, when we are induced by our thoughts to do to others what we would not like someone to do to us (See chapter 28, item 15 & chapter 15 item 10).