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"It is through conscience that human beings see and recognize the demands of the divine law. They are bound to follow their conscience faithfully in everything they do."
Religious Liberty, no 2.

Summa Contra Gentiles

by Thomas Aquinas

Book III, Chapter 122


Thomas AquinasIn this chapter Thomas discusses the morality of fornication.

Since Thomas bases his argument on the legitimacy or not of "the emission of male seed", the text is at times quoted to prove the alleged sinfulness of using contraceptives in marriage.

Such conclusions are, however, inadmissable because (1) Thomas did not know the correct biological facts and (2) he did not know the circumstances of marriage in our own times. See below for a fuller commentary.

[1] From the foregoing we can see the futility of the argument of certain people who say that simple fornication is not a sin. For they say: Suppose there is a woman who is not married, or under the control of any man, either her father or another man. Now, if a man performs the sexual act with her, and she is willing, he does not injure her, because she favors the action and she has control over her own body. Nor does he injure any other person, because she is understood to be under no other person’s control. So, this does not seem to be a sin.

[2] Now, to say that he injures God would not seem to be an adequate answer. For we do not offend God except by doing something contrary to our own good, as has been said. But this does not appear contrary to man’s good. Hence, on this basis, no injury seems to be done to God.

[3] Likewise, it also would seem an inadequate answer to say that some injury is done to one’s neighbor by this action, inasmuch as he may be scandalized. Indeed, it is possible for him to be scandalized by something which is not in itself a sin. In this event, the act would be accidentally sinful. But our problem is not whether simple fornication is accidentally a sin, but whether it is so essentially.


Great!Basic rule: what is good for human beings is OK.

[4] Hence, we must look for a solution in our earlier considerations. We have said that God exercises care over every person on the basis of what is good for him. Now, it is good for each person to attain his end, whereas it is bad for him to swerve away from his proper end. Now, this should be considered applicable to the parts, just as it is to the whole being; for instance, each and every part of man, and every one of his acts, should attain the proper end. Now, though the male semen is superfluous in regard to the preservation of the individual, it is nevertheless necessary in regard to the propagation of the species. Other superfluous things, such as excrement, urine, sweat, and such things, are not at all necessary; hence, their emission contributes to man’s good. Now, this is not what is sought in the case of semen, but, rather, to emit it for the purpose of generation, to which purpose the sexual act is directed. But man’s generative process would be frustrated unless it were followed by proper nutrition [i.e. in the mother's womb], because the offspring would not survive if proper nutrition were withheld. Therefore, the emission of semen ought to be so ordered that it will result in both the production of the proper offspring and in the upbringing of this offspring.

  error alert!

Alert 1! Biological error on conception. See below!

[5] It is evident from this that every emission of semen, in such a way that generation cannot follow, is contrary to the good for man. And if this be done deliberately, it must be a sin. Now, I am speaking of a way from which, in itself, generation could not result: such would be any emission of semen apart from the natural union of male and female. For which reason, sins of this type are called contrary to nature. But, if by accident generation cannot result from the emission of semen, then this is not a reason for it being against nature, or a sin; as for instance, if the woman happens to be sterile



[6] Likewise, it must also be contrary to the good for man if the semen be emitted under conditions such that generation could result but the proper upbringing would be prevented. We should take into consideration the fact that, among some animals where the female is able to take care of the upbringing of offspring, male and female do not remain together for any time after the act of generation. This is obviously the case with dogs. But in the case of animals of which the female is not able to provide for the upbringing of offspring, the male and female do stay together after the act of generation as long as is necessary for the upbringing and instruction of the offspring. Examples are found among certain species of birds whose young are not able to seek out food for themselves immediately after batching. In fact, since a bird does not nourish its young with milk, made available by nature as it were, as occurs in the case of quadrupeds, but the bird must look elsewhere for food for its young, and since besides this it must protect them by sitting on them, the female is not able to do this by herself. So, as a result of divine providence, there is naturally implanted in the male of these animals a tendency to remain with the female in order to bring up the young. Now, it is abundantly evident that the female in the human species is not at all able to take care of the upbringing of offspring by herself, since the needs of human life demand many things which cannot be provided by one person alone. Therefore, it is appropriate to human nature that a man remain together with a woman after the generative act, and not leave her immediately to have such relations with another woman, as is the practice with fornicators.


error alert!Alert 2! Biological error on the purpose of intercourse.

error alert!Alert 3! Biological error on the nature of women.

[7] Nor, indeed, is the fact that a woman may be able by means of her own wealth to care for the child by herself an obstacle to this argument. For natural rectitude in human acts is not dependent on things accidentally possible in the case of one individual, but, rather, on those conditions which accompany the entire species.

[8] Again, we must consider that in the human species offspring require not only nourishment for the body, as in the case of other animals, but also education for the soul. For other animals naturally possess their own kinds of prudence whereby they are enabled to take care of themselves. But a man lives by reason, which he must develop by lengthy temporal experience so that he may achieve prudence. Hence, children must be instructed by parents who are already experienced people. Nor are they able to receive such instruction as soon as they are born, but after a long time, and especially after they have reached the age of discretion. Moreover, a long time is needed for this instruction. Then, too, because of the impulsion of the passions, through which prudent judgment is vitiated, they require not merely instruction but correction. Now, a woman alone is not adequate to this task; rather, this demands the work of a husband, in whom reason is more developed for giving instruction and strength is more available for giving punishment. Therefore, in the human species, it is not enough, as in the case of birds, to devote a small amount of time to bringing up offspring, for a long period of life is required. Hence, since among all animals it is necessary for male and female to remain together as long as the work of the father is needed by the offspring, it is natural to the human being for the man to establish a lasting association with a designated woman, over no short period of time. Now, we call this society matrimony. Therefore, matrimony is natural for man, and promiscuous performance of the sexual act, outside matrimony, is contrary to man’s good. For this reason, it must be a sin.

  error alert!

Alert 4! Biological error on male semen. See below!

[9] Nor, in fact, should it be deemed a slight sin for a man to arrange for the emission of semen apart from the proper purpose of generating and bringing up children, on the argument that it is either a slight sin, or none at all, for a person to use a part of the body for a different use than that to which it is directed by nature (say, for instance, one chose to walk on his hands, or to use his feet for something usually done with the hands) because man’s good is not much opposed by such inordinate use. However, the inordinate emission of semen is incompatible with the natural good; namely, the preservation of the species. Hence, after the sin of homicide whereby a human nature already in existence is destroyed, this type of sin appears to take next place, for by it the generation of human nature is precluded.


[10] Moreover, these views which have just been given have a solid basis in divine authority. That the emission of semen under conditions in which offspring cannot follow is illicit is quite clear. There is the text of Leviticus (18:27-23): “You shall not lie with mankind as with womankind... and You shall not copulate with any beast.” And in 1 Corinthians (6:10) : “Nor the effeminate, nor liers with mankind... shall possess the kingdom of God.

[11] Also, that fornication and every performance of the act of reproduction with a person other than one’s wife are illicit is evident. For it is said: “There shall be no whore among the daughters of Israel, nor whoremonger among the sons of Israel” (Deut. 23:17); and in Tobit (4:13): “Take heed to keep Yourself from all fornication, and beside Your wife never endure to know a crime”; and in 1 Corinthians (6:18): “Fly fornication.”


[12] By this conclusion we refute the error of those who say that there is no more sin in the emission of semen than in the emission of any other superfluous matter, and also of those who state that fornication is not a sin.


Note Thomas comes to the same conclusion in his commentary on Peter Lombard:

“Those who use poisonous drugs (venena) for sterility are not spouses but fornicators” (Peter Lombard). Although this sin is grave and to be classified as a crime and against nature; for even animals desire to conceive (expectant fetus); yet it is less than murder because conception could have been prevented in some other way …

(Conjugal relations) are not to be changed to a use contrary to nature” (Peter Lomard). Marital relations are contrary to nature when either the right receptable or the proper position required by nature is avoided. In the first case it is always a mortal sin because no offspring can result, so that the purpose of nature is completely frustrated (Unde totaliter intentio naturae frustratur). But in the second case it is not always a mortal sin, as some say, though it can be the sign of a passion which is mortal; at times the latter can occur without sin, as when one’s bodily condition does not permit any other method. In general, this practice is more serious the more it departs from the natural way (In Libros Sententiarum, IV, 31, 2, 3).


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Assessment of Thomas' Argument

He bases his premises on presumed facts

Thomas is right in basing his conclusions on the facts as he thought he knew them. In this he consequently follows the principle he explained in the Summa Theologica that the application of Natural Law (deciding what is and what is not in harmony with Natural Law) depends on our human reason.

However, Thomas was ill-informed about the facts

1. BIOLOGICAL ERROR ONE - the physics of conception (see § 4 above)

Thomas followed Aristotle and other Greek authors who believed that the future person was contained in the male seed, with the mother only providing a fertile soil in which to plant the seed. See his description of conception. Read also:

(a) Aristotle's writings.
(b)The excellent article by Kim Power, “Of godly men and medicine: ancient biology and the Christian Fathers on the nature of woman”.

2. BIOLOGICAL ERROR TWO - the purposes of intercourse (see § 6 above)

Thomas rightly stressed the need for future offspring to receive support and education, but he draws his facts from species of animals and birds that stay together out of instinct. Thomas did not observe that, while animals are only 'in heat' and thus receptive for sexual union during short periods, human beings retain a permanent capacity for making love. This indicates that sexual union has another principal purpose, namely bonding that endures.

The Second Vatican Council proclaimed a vision of sexual love based on our modern understanding.

§ 49. The biblical Word of God several times urges the betrothed and the married to nourish and develop their wedlock by pure conjugal love and undivided affection. Many people of our own age also highly regard true [sexual] love between husband and wife as it manifests itself in a variety of ways depending on the worthy customs of various peoples and times. This [sexual] love is an eminently human one since it is directed from one person to another through an affection of the will; it involves the good of the whole person, and therefore can enrich the expressions of body and mind with a unique dignity, ennobling these expressions as special ingredients and signs of the friendship distinctive of marriage. [Note: the focus, even in sexual love, is not on the body but on the whole person!]

§ 50. Marriage and conjugal love are by their nature ordained toward the begetting and educating of children . . . Hence, while not making the other purposes of matrimony of less account, the true practice of conjugal love, and the whole meaning of the family life which results from it, have this aim: that the couple be ready with stout hearts to cooperate with the love of the Creator and the Saviour . . . Marriage to be sure is not instituted solely for procreation; rather, its very nature as an unbreakable union between persons, and the welfare of the children, both demand that the mutual love of the spouses be embodied in a rightly ordered manner, that it grow and ripen. Therefore, marriage persists as a whole manner and communion of life. [Note: the union between partners is recognised as another important aim of marriage.]
VATICAN II, The Church in the Modern World (Gaudium et Spes), 7 December 1965.

3. BIOLOGICAL ERROR THREE - the nature of women (see § 6 above)

Again, following Aristotle, Thomas considered women to be physically inferior to men.
* Lacking generative power, each woman is born as a deformity, a misfit of nature.
** Every woman is born through an accident of nature.
*** A man is by nature ordered to intellectual activity, a woman is not.
**** Therefore women are by nature subject to men.
Read an overview of Thomas' ideas about women's nature here.

4. BIOLOGICAL ERROR FOUR - the nature of male seed (see § 8 above)

Thomas treats the loss of male seed as almost equal to murder. The reason is that he considered male semen as virtually identical with the future child. Here again he borrows his knowledge from the classical world.

To understand this concept by the ancient Greeks and Romans, shared by the Fathers of the Church and scholastic theologians, please study the five diagrams that outline this view:
i. Spirit over matter
ii. Male over female
iii. Conception and birth of a male child
iv. Conception and birth of a female child
v. The sin of wasting male seed

Thomas's conclusions in this chapter are therefore faulty.

Thomas himself teaches that the detailed conclusions on Natural Law depend on how our human reason perceives the facts. See his essay in the Summa Theologica.