Dermot Casey, a Jesuit clergyman, published a book on scruples (i.e., obsessions and compulsions) in 1948.
Scruples are a torture, because they involve the mind in a vicious circle of fear.... Scruples are specially painful also in that the persons afflicted are those who are naturally desirous of intellectual clarity, to whom doubt is specially repugnant, and whose wills feel the urge of striving after higher moral perfection.
The person afflicted with scruples tries to fight free from his anxious doubts; he brings all his reasoning power into play in the effort to argue himself out of his scruples. The unfortunate thing is that, left to himself, his defence against them is the more fruitless the more effort he employs. His mental struggles to rid himself of his absurd doubts not only remain ineffective but aggravate his trouble. He is caught in a quicksand into which he sinks deeper the more he struggles to free himself....
A girl who is most pure finds a bad thought in her mind, and she rejects it in horror. Then she wants to be sure she did not give way to it, and she evokes the same thought in trying to judge whether or not she gave consent. This only serves to make the bad thought more vivid than the first time; she rejects it through fear of yielding to it and recalls it through fear of having consented, and she must examine it now for the two occasions instead of the one.
As regards doubts about the present: another says, "I am saying these three Hail Mary's as my sacramental penance; that is my intention now; it is for my last confession," etc., and in a quarter of an hour she cannot be sure what her intention was, nor whether she has said the Hail Mary's, nor even whether that was given to her for her penance.
These are extreme cases, but they are by no means infrequent. Those who suffer from natural, that is, non-religious scruples, sometimes think that they are on the road to insanity, and are reluctant to approach a nerve specialist or an alienist [psychiatrist], because of the stigma of being thought "half-insane." Those with moral scruples are in a somewhat better case, for they will commonly put their troubles to a confessor, obtaining, if not a cure, at least some relief....
The psychological explanation of the inner nature of scrupulosity which is by far more generally accepted today may be termed the psychasthenic theory.... The mental condition of psychasthenia, first described by Professor Pierre Janet, is to be considered as the essential subjective cause of scrupulosity. Now psychasthenia is a deficiency or weakness of general vital or psychical energy.... Scrupulosity is explained by the theory as a lowering of the psychical force to such a degree that the person becomes unequal to accomplishing certain mental acts. The mental acts which are affected first and foremost are the practical judgments relating to some one virtue, or some one moral precept, since these are the acts which come hardest to the person in question, requiring the highest degree of psychical force. The impossibility of forming these practical judgments results immediately in a chronic state of practical doubt, essentially a subjective doubt, and therefore without sufficient objective grounds to make it appear reasonable to the mind. The doubt comes from within, not from any lack of objective evidence, or from the presence of real objective evidence to the contrary, and this explains why all the efforts of the scrupulous person to resolve his doubt by new evidence remain fruitless.