Hi, Laura —

You ask important questions.

I have taken psychology courses in college and I know first hand how a Catholic can be confused as to how to fit things together and what is true and right. While to some, Freudian categories might be helpful to make intelligible what's going on in the human mind, I like to stick with the language of the Church.

Your first question relates to human freedom. The Church teaches that in order to be guilty of sin one must have full knowledge that the act is wrong and must freely consent to doing the act.

• Can mental illness, or even force of habit, compulsion, or addiction lessen one's freedom in choosing the act?

Yes. Our lower nature (Freud's Id), which includes the sensual appetites — concupiscent and irascible in Thomistic language — needs to be properly governed by our higher nature (intellect and will). With the help of grace, our mind or intellect can know the truth and our will can choose the good but, as you say, and as St. Paul states in Galatians 5:17, the tainted flesh militates against the good at times. It's an effect of the imbalance of human nature due to original sin.

To answer your question directly, if a person has mental illness that impairs his ability to reason, or it diminishes his free will to choose, then he would not be (fully) guilty for the act(s) he commits — to the degree he does not have knowledge that it is wrong or freedom to resist it.
To what degree this lack of knowledge or freedom is present within each individual, only God knows. We can guess but only God can truly judge such people's culpability.

Question two is similar and the same principle would apply. If the brain-damaged person lost their personal freedom to resist their tainted lower nature due to the brain's damage then they would not be guilty. The degree to which they are still free, however, only God really knows. Suicide victims might possibly be innocent of their act, depending on the soundness of their mind.
Again, these factors may result in lack of freedom but only God can judge that rightly.

It is possible that a person who has a rotten personality due to chemical imbalance or brain damage could have a beautiful soul. I'm not saying it's probable, but it is possible. Our soul depends on our brain and other bodily organs to express itself. If the brain is unhealthy, the personality can be unpleasant. However, we don't fully understand the interplay between the brain and the soul; whether the brain always first affects the soul or if the soul affects the brain.

• Could I, by my own free will, alter my own brain chemistry?
• Can I be guilty of causing my own mental illness?

It's a mystery, but similar to the problem of alcoholism — as a young man he is guilty for freely choosing to drink so much as to acquire the addiction, but once addicted, he may not be morally culpable for giving into his weakness at times. That argument for mental illness is less evident, but at least in the realm of possibility.

Multiple personality disorder is mysterious to both medical science and theology. I've heard of theories as to its cause, but what exactly happens inside the person is not fully known. In ancient times, the presumption would probably have been that there was demonic activity present. We shouldn't rule that out, nor should we rule out that it might strictly be the result of trauma as a child resulting in a deep-seated defense mechanism.

One thing we can say is that there is only one human soul involved, for the soul is the form of the body. But that doesn't mean that malevolent spirits cannot be involved in some way or another.

Severe mental retardation is a problem with the brain. Again, the soul needs the brain to function properly in order to develop into the personality it was naturally ordered to be. It is a defect in nature, as all of us have in some way or another.

Here's the bigger picture of what we must keep in mind:

1. To be human means to have a spiritual soul, and the powers of the soul are intellect and will — from which we may know the truth and choose the good.
   
   
2. From the beginning, moral evil gave rise to physical evil — Adam's sin brought imbalance, defect, and death to human nature, and all who are human.
   
   
3. These defects might impinge on the soul's ability to know the truth and freely choose the good if there is brain disease. To the degree it impinges on the power of the soul to freely choose, is the degree that we may not be culpable for sin.
   
   
4. God offers us natural opportunities and supernatural grace to overcome our weaknesses to become holy — despite our natural limitations.
   
   
5. Just because a brain-diseased individual may not be culpable for his actions does not mean that we must be a doormat for abuse: physical, psychological or verbal. Protecting one's self from harm is, most of the time, a noble thing, even when the perpetrator is not responsible for his acts.
   
   
6. At the resurrection on the last day Christ will raise our mortal defective bodies and glorify them if have attained salvation. There will be no more defect, weakness, or death for all eternity. Purified souls will act in and through glorified bodies in the full presence of God; and perfect fulfillment, which cannot be attained here on earth, will be experienced without end.

Feel free to follow up.

Peace,

Paul