The "Perverted Faculty" Argument in Natural-Law Theory

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    Dr. Casey, I would like to discuss an argument commonly used by Aristotelian-Thomistic natural-law theorists against non-procreative sex:

    1. The sexual faculty is one of my natural faculties.
    2. One of the functions of my sexual faculty is procreation.
    3. I should never use one of my natural faculties in a way that prevents one of its functions from being realized.
    4. Therefore, I should never use my sexual faculty in a way that prevents procreation.

    First of all, did I correctly state the traditional argument (as propounded by, e.g., Edward Feser)? If so, I want to deny the third premise.

    It does not seem to be always wrong to prevent a natural faculty from fulfilling its function. For instance, suppose that a wild animal has the natural faculty of sensing heat at a distance, the function of which is to find other animals to prey upon. Surely it would not be wrong to try to distract the animal from me (and save myself from being eaten) by placing heat sources at a distance from me, thereby preventing the animal’s heat-sensing faculty from realizing its function.

    One might respond that stricly speaking, premise 3 only claims that it is wrong to prevent the fulfilment of the function if this is done by exercising the faculty in a specific way, but not if the function is frustrated without any modification of the way the faculty is usually exercised. But I fail to see the moral relevance of this distinction. And Catholic natural-law theorists certainly want to condemn contraception in all cases, even when it is done by swallowing a pill (which is not itself an exercise of the sexual faculty).

    It might be significant that premise 3 only talks about the frustration of one’s own natural faculties, not of other people’s faculties. I think the moral relevance of this distinction has to be related to the role my own (but not other people’s) natural faculties play in the pursuit of my flourishing/happiness/eudaimonia. I have my natural faculties precisely because they contribute to my flourishing by fulfilling their functions. Since I care about my flourishing, I also have reason to care about my natural faculties fulfilling their functions to the extent this is conducive to my flourishing. So the natural-law theorist has to argue that my natural faculties’ being prevented from fulfilling their functions will in every instance diminish my flourishing.

    Although this claim is plausible with regard to the natural faculty of breathing, matters seem to be different with regard to the sexual faculty. I can have a large family even if my sexual faculty is occasionally prevented from fulfilling its function. Indeed, since extremely large families with hundreds of children are clearly undesirable, it seems that human flourishing not only permits, but even requires that the exercise of the sexual faculty should not always result in procreation, but only in a certain percentage of cases (and this is what happens even with perfectly healthy and fertile couples). It is not clear to me how slightly lowering that percentage by, say, occasionally using contraceptives is contrary to human flourishing.

    How would an Aristotelian-Thomist respond to my points? How would you respond to them, and what is your opinion of the perverted-faculty argument?


    David: You raise some very interesting questions on a topic I haven’t thought about for quite some time. If you don’t mind, I’ll take a little time to think about them and then get back to you.

    Before I do, remember there are two ways to criticise an argument: 1. dispute its validity (claim that even if its premises were true, the conclusion wouldn’t follow); and 2. dispute its soundness (grant its validity – at least for the sake of the argument – but dispute the truth of one or more of its premises). You’ve asked about the shape of the argument (validity) and disputed its soundness by questioning premise 3.

    Thanks for letting me know that you opened up a new thread.


    Thank you very much! I am looking forward to your answer (and even more to your new course on the history of political thought).


    David: Let’s take it that your argument is valid as it stands. That being so, if the premises are true, the conclusion would have to be true also. The critical premise, as you note, is premise 3. This, in one form or another, is at the heart of what has come to be called the Perverted Faculty Argument (henceforth PFA).

    It’s clear that Natural Law (NL) theorists are committed to a teleological account of ethics. Put very simply, human beings are certain kinds of beings (not just formless masses of cellular material) and we can give a broad account of those activities that genuinely contribute to human flourishing and those that don’t. So, a life lived without friends is, in general, not one that could be said to be flourishing. So too a life with a near complete absence of physiological necessities – foods, drink. However, even if all this is so, there is no single prescription for the good human life.

    Now all NL theorists invest human nature with a normative dimension. Earlier NL theorists tend to make use of the PFA in some form or another. However the difficulties with this arguments are well known. If it applies to sexual functions, does it apply to all other functions? If not, why not? What’s so special about sexuality? A hammer may have as its function the driving of nails but there doesn’t seem to be anything essentially problematic about using it to prepare steak for frying. And so on, and so on. The scholarly journals and the web are replete with discussions and examples.

    The New NL theorists have attempted to give an account of NL theory that is not open to the usual objections but, as much criticism of their position has indicated, it’s often not clear to what extent they retain a conception of human nature as normative. If you liberate ethics completely from the teleology of our embodied condition, it seems as if there is nothing to prevent us from characterising our acts in any way that we choose. (I’m not saying, by that way, that this criticism is correct or that the New NL theorists don’t have a response….)

    Despite all this, many people, myself included, retain a core intuition that as we are essentially embodied creatures, the teleology of our physical natures cannot but be relevant to the fulfilment of our ends and hence ethically relevant.

    In my earlier academic life, I published an analysis of St Thomas’s account of the morality of action in the context of lying which pointed to some difficulties in that account. You can find this at:
    Some years later, I published another piece, again analysing ethics from a broadly teleological perspective. You can find this at:

    So, where does that leave us or, more specifically, me? The PFA, while it gives expression to the basic intuition that our embodied condition and its intrinsic teleology is ethically significant, does so in a way that leaves it open to strong, perhaps insuperable, objections. The New NL theory isn’t open to these particular objections but has its own problems. If all this is so, can any reasonable case be made for the preservation of traditional sexual ethics that escapes the PFA objections and what tradition NL theories see as the limitations of the new NL theory? [I should say here that I have the utmost respect for those, such as Edward Feser and Janet Smyth, who have attempted to give reasoned accounts that are, broadly speaking, within the traditional NL camp.]

    Short answer – I don’t know, but it’s worth trying. Here you can find someone attempting to do just this.
    I don’t necessarily endorse this particular argument but I simply offer it to you as an example of where the discussion has moved to.

    I realise that you may well find all that I have said most unsatisfactory. I don’t find it particularly satisfactory myself and would like to be able to offer you a knock-down drag-out argument that settled everything conclusively. However, it should be remembered that the conclusion of an invalid or unsound argument isn’t necessarily false – it just hasn’t been proved by that argument. Moreover, we believe many things to be so without our being able to provide any argument for them at all.

    This discussion takes us well outside the scope of logic which is primarily concerned with the establishment of validity/invalidity. But it also illustrates clearly the value of logic in enabling us to focus our critical attention on those parts of an argument that are essential to its soundness.

    Best wishes,



    Dr. Casey, thanks for your reply. I agree that morality has to be based on human flourishing and that what counts as human flourishing depends on human nature and, in particular, on the biological functions of our natural faculties (just not in the way the perverted-faculty argument assumes). And I also agree that we are sometimes justified in holding moral beliefs based on our moral intuitions, just as we are sometimes justified in holding aesthetic beliefs based on our sense of beauty. However, the more controversial a (moral or aesthetic) opinion is, the more it is in need of argumentative justification.

    Thank you for pointing me to the article by Charles Capps. I like his arguments against classical and “new” natural-law theories and his proof that the sexual drive is naturally directed towards the conjugal act (i.e., a sexual act which occurs within a loving relationship and tends to result in children who are then lovingly raised by both parents). But his own defense of traditional sexual morality is unconvincing; if I am not mistaken, it goes roughly as follows:

    1. Altruism (self-giving love) is the sum and essence of morality; i.e., we should always behave as altruistically as possible.
    2. The conjugal act is more altruistic than other kinds of sexual acts.
    3. Therefore, the conjugal act is the only permissible kind of sexual act.

    Premise 2 is plausible (given Capps’s definition of “conjugal act”), but you don’t need to be an Objectivist to see that premise 1 is highly problematic (and inadequately defended by Capps). Self-sacrificing love is just one of many human goods; consequently, there can be an excess of it (such as when someone sacrifices his life to spare a stranger a small inconvenience).

    But even if premise 1 were true, the argument would be invalid (and here we return to logic). The premises merely show that the conjugal act is to be preferred over other kinds of sexual acts when both options are available.


    David: Your critique of Capps’s argument (on the succinct formulation of it you present) is, I think, correct. The conclusion validly derivable from premises 1. and 2. would be something like “the conjugal act is the more permissible kind of sexual act” which is not the conclusion Capps would like to reach. (To be absolutely certain, we should translate the argument into proper logical form but, for the moment, we can consider it more informally.)

    But even if the argument as stated were valid, its soundness would be a matter of dispute for, as you correctly note, premise 1 is highly problematic. Most moral theories distinguish between our moral duties and acts of supererogation. So, for example, we might reasonably be taken to have a moral (not legal) duty to render assistance to people in dire need where our efforts involve little or no risk or expense to us, let’s say by throwing a lifebuoy to someone in danger of drowning. But it would be a different matter to say we have a moral duty to dive into a torrent to effect such a rescue. It might be admirable and commendable so to do but failure to do so could hardly be regarded as a failure to carry out one’s moral duty.

    Best wishes,


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