(a) If the case is difficult relatively (i.e., in view of the training or lack of education of the person studying it), there can be invincible ignorance, at least for a time. Examples: Jepthe, according to St. Jerome, appears to have been invincibly ignorant that it was not lawful for him to slay his daughter. Being a soldier and living in a rude age, he perhaps did not appreciate the sacredness of human life. Unlettered persons might conceivably think in good faith that it is not wrong to commit perjury in order to help one in danger, to steal in order to pay debts, to think evil if there is no intention to fulfill it, to do what the majority do or what is tolerated, etc.
(b) If the case is difficult absolutely (i.e., in view of the matter itself, which is complicated and obscure), there can be invincible ignorance, even for a long time. Thus, it is so difficult to settle many problems pertaining to justice (i.e., to the application of the Seventh Commandment) that we find professional theologians who take opposite sides, or admit that, speculatively speaking, they do not know where the truth lies.
324. The Natural Law can never be erased from the hearts of men. (a) In abnormal circumstances only, as when the general power of reasoning has been weakened or lost, can the Natural Law be forgotten. Thus, to a degenerate who becomes violently insane murder and other crimes may appear as good acts. But no community could govern itself by the standards of madmen and long survive. (b) In normal circumstances (i.e., as long as the general power of reasoning remains unimpaired), the Natural Law cannot be forgotten, as far as its general principles or immediate conclusions are concerned, although it may be overlooked or lost sight of when it is applied to particular cases, or when remote conclusions are deduced from it.
325. As long, therefore, as a body of men remain sane, even though they be uncivilized or addicted to crime, they cannot become oblivious of the Natural Law. (a) The general principles (“good is to be done,” “evil is to be avoided”) cannot vanish from the mind, although, in particular affairs, anger, pleasure, or some other passion may prevent men from thinking about them. Thus, when the mob spirit takes hold of a crowd, it becomes intent only on violence or revenge, and gives no thought to conscience. (b) The secondary precepts, such as those contained in the Decalogue, cannot be obliterated from the mind, although in applying them to concrete situations a people may go astray.
There are many examples of laws, both ancient and modern, which permitted or commanded, for particular cases, things contrary to the current application of natural precepts. Thus, the Spartans and the Romans ordered the murder of infants who were weakly and of slaves whose master had been killed. Some ancient races encouraged robberies committed beyond the boundaries of the states, and savage tribes have been found who had the practice of putting to death parents who were aged or infirm.
326. The causes of wrong applications of the Natural Law are the following:
(a) Some causes are involuntary. Thus, the correct application may be difficult, as when more than one moral principle has to be considered and applied; or, if the case is not difficult, the person who makes the application may be mentally undeveloped, or his mind may be blinded on account of his bad education or environment. Examples: The races who saw no infamy in robbery committed against their neighbors, lived in a wild age when such acts of violence seemed necessary as measures of self-protection. The savage killed his aged parents, because to his untutored mind this seemed an act of mercy.
(b) Some causes are voluntary, such as neglect of the truth, vicious habits, etc. Examples: St. Paul blames the pagans for their idolatry, because they had darkened their own minds about God. Pirates and bandits who came to regard violence as necessary for their own defense were responsible for their state of mind, inasmuch as they had chosen a life of crime.
327. Transgression of Natural Law, therefore, is not imputable as formal sin if it is not voluntary. Hence: (a) lack of knowledge excuses, when ignorance is involuntary (e.g., those who have not the use of reason, as infants and the unconscious; children and others mentally undeveloped who cannot grasp the meaning of some precept; educated persons who are unable to get a right solution of some knotty problem of morals, etc.); (b) lack of consent excuses in whole or in part (as when one acts through fear).