One reason why the recent Vatican document regarding homosexual priests has been so poorly received among some circles of Westerners is the fact that, as a culture, we no longer recognize the causal structure of the world that the Church has so carefully illuminated for us over the centuries.
What do I mean by "causal structure of the world"? The answer is fairly complicated, but it explains exactly why the Church's position (a) makes a great deal of sense and (b) will probably not be accepted by folks outside the Tradition any time soon. In this post I want to do two things. First, I want to explain the philosophical foundation of the Church's understanding of what I'm calling the "causal structure of the world." This aspect of the post will be largely historical and--I'm afraid for some--rather boring. Second, I want to show how this philosophical foundation upholds the Church's teaching not only on homosexuality, but on a great many other things--indeed, most things--and what aspects of this foundation are rejected by contemporary Western culture. This part of the post will be theoretical rather than historical, but it will probably still be plenty boring.
1. The Causal Structure of the World
The idea that the world is causally structured actually antedates the Christian church--it can be traced back at least as far as Aristotle. Take any given entity, X, and ask the question, "What is it that makes X the kind of thing it is?" Aristotle said that we can answer that question in four different but inextricably connected ways.
Let's use a concrete example to help illustrate how these ways constitute a kind of "causal structure." Let our candidate entity, X, be a hammer. The first thing to notice about a hammer is that it must be made out of the right stuff, the right sort of material. A solid chocolate hammer might be delicious, but it will be useless for hammering in nails, which is what people ordinarily want hammers for. What is needed is a nice solid material for the head, and something suitable for the handle. Steel and wood are nice candidates.
So if you had two items before you, one made of chocolate, the other made of steel and wood, and both shaped like a hammer, you would be right to say that only one of them is a hammer. And so one answer to the question "What causes the one to be a hammer but not the other?" is to say "The one that is a hammer is made of the right sort of material". This way of answering the question is an appeal to what is called the material cause of the entity.
But suppose you had two items before you, both of which are made out of steel and wood. One of them, let's say, was made by a toolsmith, the other is just a random pile of material that fell together on the toolsmith's floor by accident. We have a tendency to say that things like hammers are the byproduct of some "intelligent design"--that is what makes them "tools" rather than just random collections of stuff. So we have an intuition that the thing made by the toolsmith is the real hammer, that is, it really is a hammer; but the other thing is just a curiosity of some sort that may or may not look like a hammer, depending on how the pieces fell together (if we are tempted to call it a "hammer" because it really looks like one, we should say instead that it is only a hammer "homonymously"--in technical philosophical jargon homonyms are things that have a name in common [in this case "hammer"] but that are really two different kinds of things). So again, if asked "What is it that causes the one thing to really be a hammer, but not the other?" we may answer "The thing that really is a hammer was not a byproduct of chance, it was made by the right sort of hammer-maker." This is an appeal to what is called the efficient cause (here "efficient" comes from the Latin efficere, to make).
Now in the case of things like hammers, we find that form tends to follow function, that is, the shape of the thing can affect how well it is able to do what it was designed to do. So a hammer has to have a certain sort of shape if it is to be efficient at hammering in nails. You don't want a hammer shaped like, say, a coat hanger, which also might be made out of steel and wood. A screwdriver also might be made out of steel and wood, but if you were to open your tool box and see two items next to each other, one of them looking like what we call a hammer, the other looking like what we call a screwdriver, you would still know that they are not the same kind of thing even if they were made out of the same materials. You would know this because of their different shapes. If someone were to ask you how you knew that one of them was a hammer and the other not, you could answer "Because the one has the form of a hammer, the other doesn't." To answer in this way is to appeal to what is called the formal cause.
In a certain sense a hammer is also caused to be what it is--a hammer--by the fact that it is used to hammer in nails. A "hammer" made out of chocolate is not just made out of the wrong sort of material to really be a hammer--it is also not able to do what it is that hammers ordinarily do: hammer in nails. All tools have functions of this sort: a screwdriver is for driving in screws and taking them out, pliers are for gripping with great strength, a ruler is for measuring and drawing lines, etc. This is not to say that you couldn't use these tools for other sorts of things. I suppose a hammer could also be used as a paperweight, or to threaten an intruder in your house--but it seems rather obvious that what a hammer is "really for" is hammering in nails--at least that's what it was designed to do, and that is what most hammers are used for most of the time. So again, if one were asked "What causes this to be a hammer rather than something else?" one could simply ask "The fact that it is used to hammer in nails." To answer in this way is to appeal to what is called the final cause (here "final" is from the Latin finalis, pertaining to an end or goal). I'm reminded of the fact that one "artist" once put a urinal in a museum and called it "art". If someone were to actually urinate in it, he would be arrested. This is because the thing is no longer a "urinal"--it now has a different final cause than a urinal, so it is no longer really a urinal. Or something like that. I guess.
Anyway, these are the four different sorts of causal accounts that Aristotle thought could be given for any kind of entity (well, entities that he thought of as genuine entities, anyway--he didn't really think that things like hammers count as entities--but we don't need to go into that here). They can also be applied to things other than intelligently designed artefacts: they can be applied to living organisms, like human beings. Let our entity, X, now be a human being. What are the four causes of this thing being a human being rather than some other kind of being?
The material cause of a human being is the right sort of flesh, blood, bone, etc. that human beings are made of. You cannot make a real human being out of, say, marshmallow. A robot is not a real human being, even a very realist looking robot like Commander Data from "Star Trek: The Next Generation", because such things are not made out of the right materials. Now, chimps and other mammals have flesh, blood, bone, etc. that is very similar to ours, but that is not the right sort of matter either, as we will see more clearly in a moment. For now it is enough to note that if you were to take a sample of chimp flesh or bone and clone it, you would grow another chimp, not a human being.
The efficient cause of a human being is, strictly speaking, the genetic lineage, but we may say more simply that it is constituted by the parents of the human being in question. Frankenstein's monster would not count as a real human because he does not have the right sort of efficient cause.
The formal cause of a human being is not as simple to describe as the formal cause of a hammer, but to see what it is recall that form follows function: the formal and final causes are always very closely related, conceptually. What is it about human life, in general, that distinguishes us from all other forms of life? That is a question that the present cultural orthodoxy regards as absurd. We are bombarded, in our culture, with arguments and reasons intended to show us that we don't really differ from other forms of life at all, or at least not in any non-arbitrary way, that we are in fact just insignificant organisms amidst hundreds of thousands of other organisms. Our genome is 94% identical with a chimp genome, our strength is nothing compared to a tiger, our voices nothing compared to the sounds of birds, our intelligenc no different from a dolphin's, etc. etc. But of course nobody in their right mind would take this kind of crap to prove anything either philosophically or scientifically interesting about the relation between human beings and other living things. Everyone knows the difference between a human being and chimp, even if chimps have genomes very much like ours. If you were to try to mate with a chimp, then as close as his/her genome is to yours, you still would not be able to breed, and there is a reason for that: human beings are one kind of thing, chimps are another. The difference between chimps and humans is not an arbitrary, socially-constructed artefact of human categorizing, it is a biological fact. And this is true across the board.
One way of looking at how this biological difference can be conceptualized is to think in terms of how it is that human beings live their lives, as contrasted with the way that other creatures live theirs. In general it seems safe to say that we do, in fact, share many behavioral traits with other animals. Plenty of animals build homes for themselves; produce vocalizations to communicate after a fashion, either with their own kind or with other animals; reproduce sexually; use tools to obtain food or to scratch themselves or what have you; and some of them even reason, after a fashion, about how best to survive in their environment. No other animal, however, is capable of the sort of highly-symbolic, infintely recursive, theoretical speculation that human beings are capable of. No other species has discovered the Pythagorean Theorem or the Categorical Imperative. So if one were to ask, what sort of behavior is characteristic of strictly human life, what activity distinguishes a human being from other kinds of beings, what sort of capacity to act causes one thing to be a human being and not another, the answer would be: "The capacity for theoretical reasoning of this kind." That is the final cause of a human being.
And that final cause helps us to see that the formal cause is whatever it is about human structure that makes that kind of activity possible. For Aristotle (and, of course, for Christians) it was the human soul. If you don't like soul talk, you can think of it as something else, if you like: the human genome, the human brain, the species itself even. But that is what the formal cause of a human being would be: whatever it is that makes this particular collection of faculties and capacities able to engage in theoretical reasoning.
So the end, for humans, our "purpose", as it were, is a certain form of rational activity. That does not mean that all human beings are always capable of the same level of rational activity. Some are capable of great rational activity, others seem fully irrational. But that doesn't make any difference to whether something is a human being, any more than the fact that one man can lift more weight at the gym would make him more human than a weaker man. The point is that the species itself, as such, has the capacity for this kind of activity, and the fact that certain individual instantiations of the species do not actually engage in that activity under certain circumstances is irrelevant.
2. Final Causation (The Natural Law) vs. Materialist Utilitarianism
This way of looking at entities in the world is thoroughly foreign to the modern scientific worldview, which is fully materialistic in its orientation. That is, all explanations, causal or otherwise, are reducible to accounts of matter and its properties. For such folks as these, there simply isn't any such thing as a "formal cause" or a "final cause", and the efficient cause of an entity is arbitrary and unimportant. In particular, the final cause has come in for particular ire, because it smacks of "purpose", and we all know that only benighted morons believe that there is such a thing as purpose in nature.
That is, of course, a benighted and moronic view of what is meant by a final cause, but no matter, the damage is already done. Materialists want only materialistic explanations, so no talk of "ends" can be permitted (except, of course, in the case of artefacts, where the "purpose" of the thing, socially constructed though it is, is still intelligible).
But for Christians, who still maintain the Thomistic view of causation that derives ultimately from Aristotle, all things--especially human beings--have final causes, and we appeal to it as normative in some sense. That is, a normal human being is, by definition, a human being in which the four causes are as they tend to be for the most part in nature. Straying form this norm is sometimes described as "against nature", though that is meant to convey not actual strife, but merely difference from the norm established through natural causes.
The phrase "against nature" is rarely heard these days, because few like its connotations. But Christians, at least, are familiar with the phrase "objectively disordered", and that is the phrase that gives rise to a significant amount of distress in the case of the teaching on homosexuality. Homosexual desires are said to be "objectively disordered" because they represent a tendency to act contrary to the causal structure that is in place in the case of human beings. The Christian (as opposed to Aristotelian) final cause of humankind is to love God, that is, to strive to will what God wills, to the extent that we are capable of doing so (and Our Lady shows that it is possible for mere human beings to will precisely what God wills in the case of any given thing that is within our capacity to will). To will what God wills would include, presumably, the order of nature as it was put into place by God. In that order of nature, sexual relations have a single purpose with a double aspect. The purpose is reproduction, and the two aspects are biological: sexual pleasure is a motivation, and the marital bond a Sacramental sign that this activity is a part of the divinely established order of nature. To engage in sexual activity that is not consistent with this picture is said to be different from the order of nature as God has established it--it cannot result in offspring--and this is what is meant in calling it "objectively disordered". It is outside the proper order of things with respect to the ends (objectives) that it can be directed towards.
Homosexuals believe that they can have "committed, permanent" bonds of love. And that may, in fact, be the case. And there is nothing disordered about any two people loving each other deeply, exclusively, and for a long time, as long as that relationship is a friendship. The worry is that such folks will believe that the relationship that exists between them can be anything other than a friendship. The "final cause" of the relationship betweem folks of the same sex can at best be only imperfectly ordered, since however much love there is betweem them there cannot be any procreation, an essential element in marriage (again, this is not to say that any particular marriage must produce children, any more than that any particular human being must be fully rational--the requirement is only that the relationship be capable in principle of procreation).
But we live in debauched times, in which the satisfaction of biological drives takes the place of any notion of final causality. Should folks who reject the notion of final causality be ordained to the priesthood? It is hard to see why the should be, but of course they may feel "called" to the priesthood. Of course, there are some women who also feel "called", and if one accepts final causality, then one may always wonder whether such "calls" are really calls or just things that appear to be calls in some respect or other. But that is another matter. In the case of the priesthood, we must again think in terms of the Church's understanding of the causal structure of the world. The priesthood is ordered towards certain ends, one of which is the modelling of Christ himself, who wills only what the Father wills. Since the Father wills the natural order that he has put into place, anyone who wills something other than that (such as a desire for sexual relations with members of one's own sex) does not will what the Father wills. There are plenty of examples of men who have been able to overcome whatever biological drives they may have that have been contrary to what the Father wills, and the present document allows for the ordination of such men. But for those for whom it is still a struggle it seems quite salutary to regard their calling as only homonymously a calling, at least until such time as they can show otherwise.
The recent Vatican document reflects a long and venerable tradition of looking at the causal structure of the world in a certain way, a way that is orthogonal to the way that our contemporary, materialistic culture looks at it, and that is rather discouraging, since more and more young men are being formed primarily by the materialistic culture around them rather than the culture of holiness and life of the Church.