But first, the concept itself. In his landmark encyclical Veritatis Splendor, Pope John Paul the Great explicated IE thus:
Reason attests that there are objects of the human act which are by their nature "incapable of being ordered" to God, because they radically contradict the good of the person made in his image. These are the acts which, in the Church's moral tradition, have been termed "intrinsically evil" (intrinsece 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.
Now, VS was the first document of its level of authority to actually give a magisterial explication, as distinct from application, of the concept of IE. A short time before that, CCC §1761 had made a start: "...there are certain specific kinds of behaviour that are always wrong to choose, because choosing them involves a disorder of the will, that is, a moral evil." That was important because it made clearer to people that intrinsically evil acts are those of kinds that it's "always wrong to choose," irrespective of any further feature of the particular act or of any further consideration about the act. In that respect, VS was an advance. Yet perforce, its explication of intrinsice malum comes after a quite interesting explication of various associated concepts that must be understood if that of IE itself is to be understood. I highly recommend them to the reader. But further interpretation and clarification is obviously needed and ongoing.
One important clarification must begin with stressing that distinctively "moral evil" is a "disorder" precisely of "the will." Hence, to will something that is intrinsically evil is a moral evil because so willing disorders precisely the will of the agent itself. But given as much, one cannot specify what, if anything, is intrinsically evil about a physical act merely by describing its physical features. And that's because one cannot say what makes the act distinctively human, an actus humanus, merely by describing what happens when somebody initiates a chain of physical events. Rather, the "object of the human act" that makes the act intrinsically evil has to be something done intentionally by the agent, in such a way that the physical feature of the act that makes the act morally significant is precisely that which "embodies the agent's intention"—a phrase first coined by Catholic philosopher Elizabeth Anscombe in her now-classic book Intention. That is the sense in which JP2 speaks of "objects of the human act" as subject to moral evaluation. Such an object is not so much what occurs in virtue of a freely chosen act; if it were, then there could be no morally significant distinction between the foreseen and the intended consequences of acts. That in turn would rule out any principle of double effect (PDE); but PDE is regularly invoked and applied in orthodox moral theology, as it should be, even though it's not yet fully clear how to formulate PDE in such a way as to minimize its misapplication. No, the "object of the human act" is what embodies the intention of the agent, even if some of what the agent foresees as flowing from what he does is not what he intends. If and when such an object is intrinsically evil, that is because what is willed and intended is an act of a kind that disorders the will of the agent. Why is that so important?
Consider the Church's teaching that contraception is "intrinsically evil." What does that mean? Citing Humanae Vitae §14, CCC §2370 says: "every action which, whether in anticipation of the conjugal act, or in its accomplishment, or in the development of its natural consequences, proposes, whether as an end or as a means, to render procreation impossible" is intrinsically evil. The Vademecum for Confessors even says that "[t]his teaching is to be held as definitive and irreformable," which leaves confessors with no excuse for excusing contraception. Now the phrase 'whether as an end or as a means' tips us off that what's subject to moral evaluation here is what one "proposes" and thus intends to do regarding something very specific. If one has sexual intercourse that one has intentionally acted so to make sterile, then whether or not the act of intercourse (a) is or would have been sterile in fact and regardless, or (b) is wrong for some other reason, the act embodying the intention to make it sterile it is itself intrinsically evil. In that sense, contraception is the "object" of that sort of "human act," and it is that object the willing of which is a disorder of the will, regardless of what otherwise ends up happening. On the other hand, periodic continence for the purpose of avoiding conception, although can sometimes be wrong for a number of reasons, is not said to be intrinsically wrong, because it is not the sort of act which, just in itself, embodies an intention to do something which it is a disorder of the will to do intentionally. Hence, under certain conditions discussed in magisterial documents, "natural family planning" (NFP) for purposes of avoiding conception can be morally acceptable. Since one is not doing anything to make procreation impossible when it might otherwise be possible, there is no "object of the human act" that is intrinsically evil as contraception is said to be.
Nonetheless, there are two equal and opposite errors about this teaching among Catholics. The more common one, which is common for all-too-obvious reasons, is an objection to the teaching itself: it is held that given the ultimate intention involved, there is no morally significant difference between contraception and NFP. That objection is registered by progs and trads for very different reasons; if it were sound, then the Church's developed teaching would be incoherent and thus not a fit object for assent.
But the objection simply misses what is meant by saying that contraception, or indeed any other sort of act, is "intrinsically evil." To call a given sort of act intrinsically evil is not to say that the further intention with which one does it, beyond the intention it actually embodies, is unacceptable. There can be all sorts of laudable further intentions with which one does something intrinsically evil. One can, for instance, intentionally kill innocent human beings with the purpose of preventing even more deaths; that, indeed, was the precise rationale for the atom-bombing of Hiroshima and Nagasaki. But that didn't make the tactic morally acceptable according to Church teaching; quite the contrary. In Evangelium Vitae §57, the same pope who wrote VS condemned any and all "direct, voluntary killing of an innocent human being" as "gravely immoral" regardless of any further intention one might have for doing such a thing. Similarly, what's intrinsically evil about contraception is not the further intention to avoid conception—which can, according to the 20th-century popes, be morally responsible—but rather the intention actually embodied in the act of contraception itself, i.e., to "render procreation impossible." Now, just why that is supposed to be intrinsically evil, apart from any further intention-with-which it is done, is unclear to a great many Catholics; and that lacuna in understanding is what accounts for the inability of some to see the moral difference between contraception and NFP. I've addressed that issue before, citing mostly JP2's "theology of the body," and shall not dilate on it here; the immediate point is that the issue is separate from that of just what sort of intentional act is said to be intrinsically evil in the first place. Only when one is clear on just what is being so condemned can one then go on to learn why it is condemned, and also why a different pattern of action with the same further intention as contraception is not intrinsically evil, even though it can sometimes be evil all the same.
The opposite error is not an objection to the teaching itself, but rather an over-rigorous interpretation of one of its premises. On this showing, the relevant "object" of the human act can be characterized as intrinsically evil not only apart from the agent's further intention in doing what he does, but apart from his immediate intention as well. For instance, if a married couple one of whose members is HIV-positive use a condom purely for prophylactic purposes, their sexual act is of a sort that is known to anti-procreative in effect even if not by intent. That's because what condoms do, when they are non-defective and used as directed, is prevent semen from being deposited in the vagina. From that, it is thought to follow that the object of the couple's sexual act, for purposes of moral evaluation, is morally unacceptable for the same sort of reason that, say, anal intercourse is unacceptable. The pattern of action is thought to be such that the sexual act in question cannot be said to have procreative significance, because it cannot bear the intrinsic relationship to procreation that HV says the conjugal act must bear. Accordingly, condomistic sex even for purely prophylactic purposes cannot qualify as a conjugal act at all, and is intrinsically evil for the same reason that sodomy is: it's an inherently non-procreative sort of act. That is held to be so even supposing that the couple would be happy to conceive if they could block HIV transmission without blocking sperm too, and even supposing that the blocking of sperm is not a means to the blocking of HIV transmission. A good example of such reasoning is this paper from Luke Gormally, a man I know personally, and one with whom I've debated this very question before on this blog.
The difficulty with that view is rather similar to one that prog theologians have often raised against what they considered the standard neo-scholastic explanation for the wrongfulness of both contraception and sodomy. That standard explanation, according to some prog apologists and theologians, was that contraception and sodomy are immoral because "unnatural," meaning that they run counter to the "natural" purpose of sexual activity: procreation. Sex that is unnatural in that sort of way was held, or thought to have been held, to be an evil object of action, irrespective of any subjective disposition of the agent, and hence irrespective of intention. Unnatural acts were thus accounted intrinsically evil. Now if that really had been the explanation, I would agree with the prog critique. What's wrong with the explanation, such as it is, is that it doesn't tell us why it is unacceptable to interrupt or depart from the course of nature in this sort of case but perfectly acceptable to do so in many others, such medicine, animal husbandry, or even cosmetology. In order to tell us that, it would have to specify how interrupting or departing from the course of nature in the case of sex embodies an intention that makes the act an intrinsically evil sort of act, i.e. an act of a sort that disorders the will when intended.
Of course I'm not at all convinced that the ancient and medieval understanding about the wrongfulness of contraception and sodomy was as ill-informed as the prog critique often makes out. It was understood better among them than among us that lust, with all its attendant disorders, increases in direct proportion to the deliberate unmooring of sex from procreation. And that should tell us something. For my immediate purpose, it tells us something that both Paul VI and John Paul II were keen to stress. What makes contraception and sodomy wrong is that they sunder a connection which is essential to our inner spiritual health, to the proper "order" of the will, thus causing us to a greater or lesser extent to treat our sexual partners as objects with which to satisfy ourselves. I've had enough experience with both licit and illicit sex to verify that for myself. But if the VS account of the objects of the "human act," is correct, then there is an intrinsically evil act here only if and when one actually intends the sundering, such that the sexual act in question embodies one's intention to break the intrinsic relationship between sex and procreation. I am not in the least convinced that condom use by married couples for the purpose of preventing infection by a lethal virus, and only for that purpose, embodies such an intention. Such activity might be wrong for other reasons, and I believe it is wrong for at least one other reason. But it is not wrong just because it is foreseeably non-procreative in effect, just as a given war is not wrong because, like all wars, it foreseeably results in the death of innocents.
To say that an action of a certain sort, such as contraception, is "intrinsically evil" is to say that it embodies an intention which it is a disorder of the will to have. Just how to identify embodied intentions and disorders of the will is the subject-matter of moral psychology. We have more than enough psychologists and moralists, but we don't have enough moral psychologists. That's because we don't have enough saints, enough lovers of God and neighbor, in the here and now. John Paul the Great was one of them. Let us learn from him.