The Manichaeans, of whom St. Augustine is rightly regarded as the most illustrious refuter, were spirit-matter dualists. More to the point, they were soul-body dualists such that they regarded the body as evil and the soul as good. They even ascribed to the body its own “will,” which they regarded as base and evil.
Believe it or not, many many people think that Christianity–especially Catholicism–has always taught this about man: that his soul is good and his body is evil. The world thinks that Catholicism regards the body and sexuality as sinful. Even my arch-nemesis The Complete Idiot’s Guide to Understanding Catholicism claimed that the Church professes dogmas such as the perpetual virginity of Mary because it regarded the body and sex as evil. In fact, the Church has constantly dealt with heresies that introduce a division into man between soul and body: from Docetism to Descartes.
Irony of ironies, the world is dualistic and Manichaean in its approach to the human person. Just look at how we justify things these days: “I can do what I want with my body.” Besides the glaring error when this statement is used to justify abortion (what you’re doing only secondarily involves your body and is primarily intended to kill another person), what’s the reasoning? This statement means “the body has nothing to do with morality.”
For example, let’s think about so-called “homosexual unions.” I say “so-called” because homosexual actions actually divide rather than unite those who engage in them: this actually follows from what advocates for their liceity themselves claim.
Why should homosexual actions and heterosexual actions be morally equivalent? The specific difference between the two (indeed, the characteristic which even gives us the words “heterosexual” and “homosexual”) is that homosexual actions do not involve sexual complementarity while heterosexual actions do. There is simply no denying this complementarity on a physical level (to speak vulgarly, even in terms of “mechanics”).
Now, what is the argument from those who hold that the two are morally equivalent? They say that sexual complementarity on the physical level is not relevant to the morality of the action, or, in general, a person’s body or his sex does not pertain to his actions in a morally relevant way. Now, every morally relevant action is an action of the person, that is, of the subject. So, what they’re saying is that the body is divided from the person. If this were true, then it would be true that homosexual relations and heterosexual relations are morally equivalent, but it would also necessarily mean that no act of sexual intercourse could involve the persons who perform it other than by a sort of merely incidental association.
Sex would then be an act of the body, but it could never be a personal act; it could never involve love. Frightful to say, but if this line of thinking is continued, then things terrible crimes such as rape would be morally equivalent to petty theft since they would be thought to involve only the body regarded as drastically foreign to oneself.
Recall, again, the Manichaeans. They did not strive to overcome the flesh because they regarded it as evil and as a separate thing from the person: they became consumed with seeking sensual pleasure.
Men who deny the goodness of the body do not make themselves angels.
John Paul II talks about this same tendency in Veritatis Splendor:
48. [ ... ] A freedom which claims to be absolute ends up treating the human body as a raw datum, devoid of any meaning and moral values until freedom has shaped it in accordance with its design. Consequently, human nature and the body appear as presuppositions or preambles, materially necessary for freedom to make its choice, yet extrinsic to the person, the subject and the human act. Their functions would not be able to constitute reference points for moral decisions, because the finalities of these inclinations would be merely “physical” goods, called by some “pre-moral”. To refer to them, in order to find in them rational indications with regard to the order of morality, would be to expose oneself to the accusation of physicalism or biologism. In this way of thinking, the tension between freedom and a nature conceived of in a reductive way is resolved by a division within man himself.
This moral theory does not correspond to the truth about man and his freedom. It contradicts the Church’s teachings on the unity of the human person, whose rational soul is per se et essentialiter the form of his body. The spiritual and immortal soul is the principle of unity of the human being, whereby it exists as a whole — corpore et anima unus— as a person. These definitions not only point out that the body, which has been promised the resurrection, will also share in glory. They also remind us that reason and free will are linked with all the bodily and sense faculties. The person, including the body, is completely entrusted to himself, and it is in the unity of body and soul that the person is the subject of his own moral acts. The person, by the light of reason and the support of virtue, discovers in the body the anticipatory signs, the expression and the promise of the gift of self, in conformity with the wise plan of the Creator. It is in the light of the dignity of the human person — a dignity which must be affirmed for its own sake — that reason grasps the specific moral value of certain goods towards which the person is naturally inclined. And since the human person cannot be reduced to a freedom which is self-designing, but entails a particular spiritual and bodily structure, the primordial moral requirement of loving and respecting the person as an end and never as a mere means also implies, by its very nature, respect for certain fundamental goods, without which one would fall into relativism and arbitrariness.
49. A doctrine which dissociates the moral act from the bodily dimensions of its exercise is contrary to the teaching of Scripture and Tradition. Such a doctrine revives, in new forms, certain ancient errors which have always been opposed by the Church, inasmuch as they reduce the human person to a “spiritual” and purely formal freedom. This reduction misunderstands the moral meaning of the body and of kinds of behaviour involving it (cf. 1 Cor 6:19). Saint Paul declares that “the immoral, idolaters, adulterers, sexual perverts, thieves, the greedy, drunkards, revilers, robbers” are excluded from the Kingdom of God (cf. 1 Cor 6:9). This condemnation — repeated by the Council of Trent” — lists as “mortal sins” or “immoral practices” certain specific kinds of behaviour the wilful acceptance of which prevents believers from sharing in the inheritance promised to them. In fact, body and soul are inseparable: in the person, in the willing agent and in the deliberate act, they stand or fall together.
Bolding by me.
St. Augustine wrote the following about Manichaeism (Confessions, Book V, Section 10):
I still thought that it is not we who sin but some other nature that sins within us. It flattered my pride to think that I incurred no guilt and, when I did wrong, not to confess it… I preferred to excuse myself and blame this unknown thing which was in me but was not part of me. The truth, of course, was that it was all my own self, and my own impiety had divided me against myself. My sin was all the more incurable because I did not think myself a sinner.