We've talked before about what is a mortal sin. One of the constituants of a mortal sin is that the sinner know that what he or she is doing is a sin. So unlike in a court of law ignorance is a defence. Technically what is ignorance? Ignorance is a lack of knowledge of something which can be known.
So if one does not know something is a sin then it is not.
But, wait a minute! Does that mean that if no one told me it is wrong to kill than I'm home free?
Not quite. Ignorance can be divided into two catagories, Vincible and Invincible. Ignorance is said to be invincible if an individual cannot rid himself of it through the exercise of moral diligence. Ignorance is vinicble if it can be cure throug the use of moral diligence.
What does that mean? It means that when making a moral decision a person must exercise a reasonable effort to attempt to discover the moral choice. Should the sin in question concern a matter of natual law then the person can reasonably be expect to know that his action are a sin.
We'll discus natural law tomorrow.