Can a horse run across a cliff?
Or can one plow the sea with oxen?
Yet you have turned judgment into gall,
And the fruit of justice into wormwood.
Amos 6:12
Much as a cliff ends the run of a horse, and oxen flounder in the sea, so also man falters in the sight of God when performing an intrinsically evil act. The tree of good and evil stood tall and ready in Paradise when God displayed it to Adam. God did not ask Adam to trim it or to plant another.
TELEOLOGICALISM, CONSEQUENTIALISM, PROPORTIONALISM. A novel basis to ascertain moral goodness and evil has been proposed during recent decades. It denies the existence of absolute moral norms. Pope John Paul II responded to this flawed opinion in the Encyclical Veritatis Splendor. The novelty maintains that intentions of the actor and foreseen consequences of proposed acts are so complex that one cannot judge that certain actions are always and everywhere and in all circumstances evil. The Pope denies this categorically:
The opinion must be rejected as erroneous which maintains that it is impossible to qualify as morally evil according to its species the deliberate choice of certain kinds of behavior or specific acts, without taking into account the intention for which the choice was made or the totality of the foreseeable consequences of that act for all persons concerned (VS 63).
Reason attests, on the contrary, that the objects of some human acts are by their very nature "'incapable of being ordered' to God because they radically contradict the good of the person made in his image. These are acts which, in the Church's moral tradition, have been termed 'intrinsically evil' (intrinsice malum); they are such always and per se, in other words, on account of their very object, and quite apart from the ulterior intentions of the one acting and the circumstances" (VS 80).
OBJECTIVE BASIS OF MORAL ACTS. That man can recognize with his mind that certain actions are always wrong, no matter what the actor intends or foresees, is expressed in Romans 3:8: "Why may we not do evil that good come of it? This is the very thing that some slanderously accuse us of teaching, but they will get what they deserve."
Thomas teaches that our human minds participate in the mind of God when we discern our moral obligations. To a question about who will teach us what is right he responds: "The light of your face, Lord, is signed upon us'. He thereby implies that the light of natural reason whereby we discern good from evil, which is the function of the natural law, is nothing else but an imprint on us of the divine light" (STh I-II,91,2; VS 42). The Pope then adds that it is reason that promulgates the natural law. We may add that, because human reason reflects the divine law but does not create it, therefore it cannot change the law. We cannot change what is in the mind of God. We cannot create valid consciences that differ from the Eternal Model.
Only God can answer the question about the good because he is the Good... He did so by creating man and ordering him with wisdom and love to his final end, through the law which is inscribed in his heart (cf. Rom 2:15), the "natural law". The latter "is nothing other than the light of understanding infused in us by God, whereby we understand what must be done and what must be avoided. God gave this light and this law to man at creation" (Saint Thomas Aquinas in Duo Praecepta Caritatis; VS 12).
When we see light in God's light, when we embrace good and avoid evil because we are His images, His thoughts and His goodness are the source of our participation. We are intellectual images of His omniscience, moral replications of His goodness. His thoughts model our thoughts, His hatred for evil molds our aversion of evil. Being guests at His wedding feast, we wear the prescribed garment and follow the rules of heavenly etiquette.
INTRINSICALLY EVIL ACTS IDENTIFIED. VS 80 identifies the following as intrinsically evil acts, quoting GS 27: "Whatever is hostile to life itself, such as any kind of homicide, genocide, abortion, euthanasia and voluntary suicide, whatever violates the integrity of the human person, such as mutilation, physical and mental torture and attempts to coerce the spirit; whatever is offensive to human dignity, as subhuman living conditions, arbitrary imprisonment, deportation, slavery, prostitution and trafficking in women and children; degrading conditions of work which treat laborers as mere instruments of profit and not as free and responsible persons: all these and the like are a disgrace, and so long as they infect human civilization they contaminate those who inflict them more than those who suffer injustice, and they are a negation of the honor due to the Creator." VS then adds contraception to the list of intrinsically evil acts, reflecting developments since GS was written in 1963. Were it written today, it would surely add sexual acts with members of the same sex.
Human society needs to observe the ban against intrinsically evil acts for its own protection. Freedom from crimes, such as murder, adultery, fornication, theft, fraud, sacrilege and so forth is the foundation of human civilization and welfare (cf. VS 15). It follows that the Ten Commandments ought to be displayed in every legislature and judiciary. They firm up the Constitutions. Jettisoning the Commandments collapses the nation, is treason, is national patricide.
Space ships reach their destination only if they follow correctly the consistent calculus of mathematics. Man lands in heaven only if, aided by God, he follows correctly the patterns of behavior that God imprinted on his mind, Acts that are intrinsically evil remain so no matter what the intentions and foresight of actors may be.