Plato answered this question by asserting that all sin is the direct result of ignorance. He asserted (through the character of Socrates in Protagoras (emphasis added):
....no wise man believes anyone sins willingly or willingly perpetrates any evil or base act. They know very well that all evil or base action is involuntary…when people make a wrong choice of pleasures and pains—that is, of good and evil—the cause of their mistake is lack of knowledge…no one who either knows or believes that there is another possible course of action, better than the one he is following, will ever continue on his present course when he might choose the better. To “act beneath yourself” is the result of pure ignorance, to “be your own master” is wisdom.For a sin to be mortal the gravely evil action must be done voluntarily (see CCC 1857), so Plato's argument amounts to a complete denial of what Catholics would later term "mortal sin." Simply put, if Plato is right, there is no mortal sin.
But, I think we all know better from our own experience. We know when something is wrong. And all too frequently we do it anyhow. Dr. Peter Kreeft explains that we do this because we are all "crazy" because "every sin is choosing misery over joy." Choosing is the operative word. Sin is not a defect in the intellect, as Plato would have it, but in the will (in the "choosing" faculty, not in the "knowing" faculty).
We sin not because we are stupid, but because we are weak. Plato seems to recognize this himself, in another one of his works - the Phaedrus, with his famous "chariot allegory." Plato compares the human soul to a chariot with two horses. The horses represent our "passions" those things we want to do naturally (have sex, eat, etc.), but that may be disordered (a man wants to view porn, a woman who wants to eat ice cream daily, etc). The charioteer is our will and it's his job to keep a tight reign over our passions, making sure they are directed only to good ends and preventing them from charging headlong after the wrong things (porn, binging on ice cream, etc.). If our charioteer (the will) is weak, we will sin frequently and grow in vice. If our charioteer is strong enough to keep a tight grip on the reigns of our desires, then we will sin less frequently and grow in virtue. Thus, it is a weak will, not a weak intellect that is the primary cause of sin.
St. Thomas Aquinas explains, in the Summa Theologiae, explains the root cause of our sins:
... the will lacking the direction of the rule of reason and of the Divine law, and intent on some mutable good, causes the act of sin directly, and the inordinateness of the act, indirectly, and beside the intention: for the lack of order in the act results from the lack of direction in the will.
|St. Thomas Aquinas|
According to Thomas, sin is caused by a lack of direction in the will. In other words, a weak will leads man to sin. Thomas goes as far as saying that because "the completeness of the voluntary sinful act appertains to the will, so that the act of the will... is already a sin." Thus, willing to do something evil, even if we can't do it because of some external cause, is already a sin because sin is primarily (although not exclusively) an act of the will.
This all can be seen directly in the account of the very first sin, the sin of Adam. Adam and Eve know, by direct revelation, that eating from the Tree of Knowledge is wrong, yet they are too weak to resist the temptations of the serpent. They aren't too stupid to avoid sin, they are too weak to persevere in the face of temptation (and Satan).
Just like Adam and Eve, we too are tempted. Jesus, in the Gospel of Matthew, chapter 18, says, "...it is necessary that temptations come." What ultimately decides whether we will fall to that temptation and sin or stay strong is our will.
For those battling a deep rooted sin, an addiction, it might seem an impossible task to ever strengthen the will enough to overcome their sinful ways. And, indeed, it is... without Christ, but with Christ we can do all things (cf. Phil 4:13).
And that is Good News!