Theory and practice of the confessional by Caspar Erich Schieler, Richard Frederick Clarke
III. In order that the excuse of moral impossibility may be pleaded it is necessary, 1, that there should be a real or probable risk of great harm; 2, that it is impossible to find another confessor to whom a full disclosure may be made without fear of this particular harm; 3, that only those sins or circumstances be kept back of which the avowal would cause harm; and finally, 4, that the confession cannot be put off.
IV. Physical impossibility might result from, 1, inculpable forgetfulness or inculpable ignorance, or only venially culpable ignorance and forgetfulness. A man who is ignorant invincibiliter et inculpabiliter that the particular act which he calls to mind is sinful, or does not know that his sin must be confessed with its number and species and circumstances changing the species, is not bound to integrity in confession; there is still less obligation on an uneducated and weak-minded penitent.
If, again, a man in examining his conscience cannot recall a past sin, or, having recalled it, forgets about it in the confessional, he is physically incapable of making a complete confession. (On this point see the preceding paragraph.) It is to be noticed, however, that in the case of gravely culpable negligence or carelessness in examining the conscience an imperfect confession is invalid; if, for example, a man through his own fault is ignorant how confession ought to be made, or was unwilling to make a careful examination of his conscience. On the other hand, one is not obliged to go to confession sooner in order not to forget past sins, though frequent confession is much to be recommended; for we are bound only to accuse ourselves of the sins of which we are conscious at the time of confession after making a diligent examination of conscience.