I spent some time on the Catholic Answers forums the other day. I have an interest in moral theology and so browsed through those forums, and was a little surprised by what I believe to be widespread confusion concerning the nature of sin. Take for example one of the most popular topics, masturbation. Over and over again the sin of masturbation was excused because of habitual or social factors. Masturbation was a sin, most maintained, but couldn't be a mortal sin because it was a habit, or the world presents too many temptations, or for some other reason.
The problem with all this moral hair-splitting and modern psychology is that the concept of sin is essentially eliminated.
What I mean is this: every outward action of a person has social or circumstantial elements because human beings are both social and contingent beings. A reasonable theology of sin must be able to incorporate this human reality without eliminated the very concept of sin. To say that an individual must have perfect voluntariness for something to be a mortal sin and to mean by this that there cannot be circumstantial or social contingencies in his outward act is the same as saying that a man cannot commit a mortal sin. Mortal sin is the preserve of Angels. I'm reminded that even the first sin had a social aspect and a temptor. Were Adam and Eve completely free, did their action represent perfect knowledge and perfect will? The very foundation of our religion demands that we be able to answer yes, and yet Eve was tempted and Adam tricked. I repeat, every action of a man has social and circumstantial components.
We can perhaps get around this if we simply bury the sin a little deeper in the man and say that whatever component of the decision which was completely free and in no way contingent on external circumstances or influences, no matter how tiny, is were sin resides. In doing this we can allow for any amount of circumstantial or social influence without eliminating sin. However, the sin of the person becomes far removed from the external action. As long as we maintain that there exists even an infinitely small amount of freedom of the individual from society and from his own psychosis, even if it is so small that it is incapable of influencing his actual external actions, there is room for sin. The point, however, is that the more we build up psychological analysis and “social construction of reality” theories, the deeper we push the concept of sin into the mysterious core of the individual.
In principle this is fine, but if we are going to think about sin this way we have to reevaluate what we mean by grave matter. The definitions that we inherit from tradition are based on a concept of the will that was much more free than modern psychology maintains. Traditionally the social and circumstantial elements in an individual’s actions where simply taken for granted. Masturbation was a grave sin not because someone had perfect freedom from social pressures; rather it was grave matter assuming the scandalous influence of the world. So, if we are going to isolate sin from society we need to isolate grave matter from society. It would no longer be coherent to talk about grave matter that involves other people or their influences. Masturbation itself ceases to be sinful matter. However, all we are really doing is shifting the sinful action into that tiny corner of the individual’s psyche that is free from influence. And so, the rebellion to lust that necessarily occurs, no matter how deep in the individual’s soul, every time he masturbates is the grave matter.
The point is we can’t have it both ways. We can’t psychoanalysis ourselves virtually out of existence and then retain the lists of grave matter from the Middle Ages. The list of grave matter sins needs to keep pace, or we need to just accept the fact that we sin because we like it and that we know that it is wrong and that we do it anyway and that this is mortal sin!

In his Confessions, St. Augustine writes of his time among the Manichean heretics:
"For it still seemed to me “that it is not we who sin, but some other nature sinned in us.” And it gratified my pride to be beyond blame, and when I did anything wrong not to have to confess that I had done wrong--“that thou mightest heal my soul because it had sinned against thee” --and I loved to excuse my soul and to accuse something else inside me (I knew not what) but which was not I. But, assuredly, it was I, and it was my impiety that had divided me against myself. That sin then was all the more incurable because I did not deem myself a sinner. It was an execrable iniquity, O God Omnipotent, that I would have preferred to have thee defeated in me, to my destruction, than to be defeated by thee to my salvation. Not yet, therefore, hadst thou set a watch upon my mouth and a door around my lips that my heart might not incline to evil speech, to make excuse for sin with men that work iniquity. And, therefore, I continued still in the company of their “elect.”"