In the comments, reader Matthew offers the following suggested definition as an alternative to those already offered on this blog and elsewhere:
"Torture is the unjust use of extreme mental or physical violence which degrades the human person. Examples of this would be the use of physical and mental violence against prisoners when the end is not justified, for reasons such as revenge, humiliation or to extract information from someone when their probable guilt to immediate threats to innocent human life are not at stake. This would not include proportionate means of physical and mental violence done by the State for the just purposes of self defense or for exacting retributive punishment, including Capital Punishment. Even the just use of physical and moral violence would be limited to the proportion of the crime committed in the classification of the State's right and obligation to keep the moral order and to exact retributive punishment. In the case of the State' right to protect innocents in the act of self defense, actions such as amputating limbs or taking the violence past the rational faculties can never be done."
Now, I love the first sentence of this definition. I agree with it one hundred percent. I find it concise, clear, and useful for practical application.
Then it turns consequentialist. That is to say, it stipulates that the end justifies the means. The end of preventing "immediate threats to innocent human life" allows us to "degrade the human person" of whomever happens to be in custody at the moment.
This same Matthew also asks for discussion of moral principles. So I will lay out my own understanding of foundational Catholic moral principles, which I hope will show exactly why consequentialism is a serious problem. I'll try not to be too long-winded.
First, I start where St. Thomas Aquinas started, and where the Catechism starts: with the purpose of human life. Our entire reason for being is to enter into that eternal communion with God which is usually called Heaven. In a word: happiness.
Our happiness entails acting according to our nature as God created us, and answering our supernatural call. Morality is the entire realm of human activity. A "moral" act is one that accords with nature and answers our vocation. An "immoral" act is one that is contrary to nature and/or to God's call.
Virtues are habits of action that support us in our natural and supernatural life. Vices are habits that reinforce the damage and stain of original sin, separating us from Life Himself.
Sin is a particular act that separates us, either venially or mortally, from God.
Law is a support to virtue in (at least) three ways: first, making known the order and purpose of creation, and of ourselves within it; second, warning us against acts that threaten our happiness; third, providing direct consequence for such vicious actions.
I know this is a cursory and abstract presentation, but I hope it is enough to keep the conversation moving forward.
Let me just leap over all the intervening steps and say, the reason torture is intrinsically evil is that it is impossible to degrade any human person and at the same time be acting in accordance with the nature God gave us, or the supernatural life to which he calls us. It is always and unchangeably sinful.