1. Acts are determined by their objects. The etymology of “object” suggests something thrown against. The object of an act is that against which or on which the act acts. The object of seeing is color. And color determines seeing; it makes seeing into the kind of act that it is. The object of hearing is sound, the object of eating is food, the object of nursing is a baby, the object of killing is a living thing. And in all these cases, the object determines the act, makes it to be the kind of act that it is, and gives it its nature.
2. For acts have natures. Just as natural things have principles of change and stability within them that direct them toward their ends— participations in the intelligence of the Creator who makes them words through which He speaks— so too do the actions of such things have intrinsic ordering principles. And just as the natures of things are that in virtue of which we can distinguish different kinds of things, so the nature of an action is that which enables us to say “this is an action of such a kind.”
3. The goodness or evil of an act is derived in the first place from the goodness of their object. Seeing is good because color is good; eating is good because food is good; nursing is good because babies are good. Killing is at least in some respect evil, since it involves the destruction of a good object. Life is good, and its destruction is in some sense evil. Good and evil have a general meaning sense here; not necessarily a moral sense.
4. Human acts, in the strict sense, are acts that proceed from reason and will. Such acts have a special kind of goodness and evil: moral goodness or moral evil. Moral goodness is a good that tends toward and contributes to happiness, the end of human life. Moral evil is an evil that tends away from that human end. Inasmuch as human acts have moral goodness or moral evil, they are called moral acts.
5. Like all acts, moral acts are determined by their objects. The object of the moral act is what an acting person chooses to do. That is, it is the action itself considered as elicited or commanded by the will. But this means that the object of a moral act is complex: it is determined by the external object of the action and by the relation that reason sees in it to the end of human life. Since the will is a rational appetite, a person cannot choose an action unless it is known by reason and presented to the will by reason as something choice-worthy, as a good.
6. The object of the moral act thus has two components. First, it has a “material” (undetermined) component: namely the external or physical reality of the action, determined by its “natural” or “physical” object. For example, “cutting open a person’s chest” is the action of cutting open determined by the object of “a person’s chest.” And second, it has a more “formal” component that determines the material component: namely the practicable goodness that reason finds in the action. For reason to find practicable goodness in an action means for it to see that action as tending toward complete goodness, happiness. For example, reason can see “life-saving surgery” in cutting open a person’s chest and understand that surgery as tending towards happiness.
7. Objects which are “materially” the same can be determined as different kinds of actions by the formal component. Thus, if a violent man cuts open a person’s chest for the sake of revenge, and a surgeon cuts open a person’s chest for the sake of saving life, they are performing different kinds of actions. “Cutting open a person’s chest” is a different moral object when reason finds “violent revenge” in it than when reason finds “life-saving surgery” in it.
8. Because the object determines the nature of an act, it is the most important reality in judging the morality of an act— more important than the ulterior consequences of an act, or the intentions of the agent with respect to such consequences, or any other circumstances of the act. The Church teaches that certain actions are intrinsically evil, merely because their objects are of themselves unable to be ordered to the end of human life. Thus, committing adultery or lying are evil on account of their objects, which cannot tend towards or contribute to the end of human life. Now, persons may well commit adultery or lie for the sake of good consequences which they expect to follow from those acts. For example, they might commit adultery for the sake of receiving shelter and support or tell a lie in order to protect themselves from danger. But in the acts themselves, they are actually destroying their happiness.
9. To understand how an evil moral object, even if good consequences are expected from it, is contrary to happiness, it is helpful to consider the distinction between making and doing, and between art and prudence. Making is causing some external thing to be. The good of making is found in the perfection of that external artifact. Doing is an action considered simply as chosen by an agent. The good of doing is found in the one doing himself. Doing is a perfection of the one doing.
10. Making proceeds from art, which is a habit that enables someone to make well. An artist’s “own act,” the “function” or “work” of an artist, is to make something well. Thus, the proper act of a cook is to cook well, and the proper act of a sculptor is to make good statues. A person can be a good cook or a good sculptor, but be a bad person.
11. Doing proceeds from prudence, which is a habit that enables a person to recognize and choose what tends towards the perfection and end of a person as a person. Now, a person’s “own act,” the “function” or “work” of a person as a person, is the “act with reason”; that is, “the acts of reason itself—such as knowledge, understanding, and reasoning— and the acts of other parts of the soul as guided by reason— the acts of the will done reasonably, the acts of the emotions guided and moderated by reason.” This act with reason is the very living of human life; human life is activity with reason. And this activity is happiness. Human doing does not relate to happiness the way the making of an artifact relates to the complete artifact. The making of an artifact is only important insofar as it brings about some good thing at least notionally distinguishable from it: the artifact. But human doing brings about happiness by being an element of happiness. Therefore, to do something in itself disordered, such as lying or adultery, is always opposed to happiness, no matter how many good consequences might flow from it. A lie cannot be perfective of the one lying because the lie is perverse in its own nature. The natural object of speech is communication of what one thinks. A lie perverts the nature of speech by using it to communicate what one does not think. Thus, a lie is essentially irrational and destructive of the happiness of the one lying. Similarly, adultery is contrary to the natural object of sexual union, and thus reason cannot find happiness in it.
12. Happiness is not some product of human activity to be produced by cunning design; it is the activities of human life itself done in a truly human, a truly rational way.
Postscript (2020): I have come to see that the sense of “moral object” that I used above is not the same as that used by St. Thomas. (Although I think that recent magisterial teaching does use “moral object” in the sense that I explain). For St. Thomas the object is not “what an acting person chooses to do,” but rather “the thing to which the action is done”. As Urban Hannon put it, “for St. Thomas, the ‘moral object’ or ‘object of the act’ is not the nature of the act you’re performing. It’s the external thing upon which you’re performing the act. Not that it is the species, but that it gives the species. It is the difference.” Thus, the object of adultery would be “a married woman” rather than “sexual union with a married woman”. The object of murder would be “a human being” rather than “killing a human being”. The object of lying would be “a falsehood,” rather than “saying a falsehood.” Hence St. Thomas says: “A moral act takes its species from two things, its object, and its end: for the end is the object of the will, which is the first mover in moral acts. And the power moved by the will has its own object, which is the proximate object of the voluntary act.” It is that proximate end, the end of a power other than the will, which St. Thomas calls the “object,” whereas recent magisterial texts use object to mean the action itself as the object of the will.
 See my paper, “Natures as Words, Contraception as a Lie,” in: Ethika Politika, September 16, 2014: https://ethikapolitika.org/2014/09/16/natures-words-contraception-lie/ (accessed May 14, 2017).
 See: St. Thomas Aquinas, O.P., Summa Theologica, Ia-IIae, q. 19; Romanus Cessario, O.P., Introduction to Moral Theology (Washington, D.C.: The Catholic University of America Press, 2001), pp. 167-170; Steven A. Long, The Teleological Grammar of the Moral Act (Ave Maria: Sapientia Press, 2007), ch. 1.
 See: Duarte Sousa-Lara, “Aquinas on the Object of the Human Act: A Reading in Light of the Texts and Commentators,” in: Josephinum 15.2 (2008), pp. 243-276.
 See: Aquinas, Summa Theologica, Ia-IIae, q. 57, a. 4; Jacques Maritain, The Responsibility of the Artist (New York: Scribner, 1960), ch. 1: Art and Morality; Charles De Koninck, “Art and Morality;” Entirely Useless, “Doing and Making,” Entirely Useless (blog): https://entirelyuseless.wordpress.com/2016/05/07/doing-and-making/ (accessed May 14, 2017).
 Edmund Waldstein, O.Cist., “The Good, the Highest Good, and the Common Good,” in: The Josias, February 3, 2015: https://thejosias.net/2015/02/03/the-good-the-highest-good-and-the-common-good/ (accessed May 14, 2017).