Steve Cooke

measuring the boundaries of our nation by the sun

Natural Law Theory and Gay Marriage


This past fortnight has seen frantic discussions on the extension of marriage to gay couples. I’ve seen repeated use of the argument against on grounds that ‘marriage is between a man and a woman for the purpose of procreation’. But why use this apparently weak argument? For one thing, it appears to outlaw marriage for infertile people , and it seems to therefore require that marriages be ended when women reach the menopause. But religious groups aren’t arguing this – so are they being inconsistent? In spite of the evidence, they say not – and the reason is routed in their belief in natural law. It’s for this reason that I’ve picked the chapter (by Knud Haakonssen) on Early Modern Natural Law from the Routledge Companion to Ethics as my reading for today.

If you recall my previous post on Plato and Socrates, I mentioned Plato’s dialogue Euthyphro. The arguments in that dialogue convincingly demolish the idea that what is morally right means what God commands (known as Divine Command Theory). The weaknesses in Divine Command Theory led to the adoption of Natural Law theory as a theory of ethics (particularly by the Catholic Church).

So what is Natural Law? The theory is that there are certain laws of nature, and things in nature have particular values and purposes (the Christian view being that these have been created by God). To determine if something is right (such as a law, or act) we ask ourselves if it is consistent with its natural purpose or the laws of nature. Probably the most famous articulations of Natural Law Theory is that provided by Thomas Aquinas – known as Thomism. Thomist thinkers were influential in the drafting of the United Nations Universal Declaration of Human Rights – so this theory is pretty pervasive in moral thought.

From a Catholic natural law perspective (I’m going to say nothing on secular natural law theories in this post) gay marriage is wrong because the natural purpose of sex and marriage is reproduction, and therefore gay sex and gay marriage are unnatural and thus wrong (along with masturbation, oral sex, and a host of other sexual practices). But, why think that the purpose of sex is purely for procreation? And on what basis is marriage thought to be natural for humans in relationships? And why should we accept the idea that using certain bits of our bodies in ways that seem to be different from their evolutionary purpose (such as our feet for playing football, our voices for singing, mouths for whistling, hands for doing handstands etc.) is morally wrong? And how should we think of bodily parts that don’t function as they should due to disability? I’m not clear that the theory has any answers to these questions.

One of the more interesting (and in my view, wrong-headed) aspects of Natural Law Theory is the idea that ethical judgements about things are made at the category level, rather than individual level. For example, we say that human beings are the rational creatures and accord them rights based on that rationality regardless of whether a particular human is rational. This idea means that a human who has no brain function should have the same rights as a fully rational moral agent. It also means that what is known as the Argument from Marginal Cases (AMC) which is often used in animal rights theory cannot find purchase. In brief the AMC points to the fact that it is inconsistent to grant rights to humans and not to non-human animals if when there are no morally-relevant characteristics that all humans and no animals possess to the relevant degree. So, if we think intelligence or rationality are the capacities that make us worthy of moral concern, then we need to explain why we should treat those humans who have them to a lesser degree than the norm with more moral consideration than those animals that have those characteristics to the same or greater degrees. Why should a Dolphin be worthy of less moral concern than anencephalic infant for example? I’ve yet to figure out why we should judge a being’s moral worth according to the category of being they are in rather than on facts about the individual being, but that’s what Natural Law theory says we should do.

Natural law theory fails because it commits Moore’s naturalistic fallacy by trying to derive ought from is. And it fails because it relies upon a questionable metaphysic that derives those moral facts from natural facts at the category rather than individual level. Additionally, Natural Law Theory says we should be able to grasp moral laws through the exercise of our reason (since this is part of our purpose as rational and free agents) – but this means that religious authorities have no special access to truth – they must supply and respond to good reasons in the same way as the non-believer. On the issue of gay marriage, I can’t say that I see this being the case – there’s a lot of irrational, bigoted and arbitrary nonsense from those opposed to letting two people who love each other publicly cement their relationship through marriage. Mini-rant over (go sign this petition for equal marriage rights).

The nice thing about reading that chapter was that not only did it explain natural law theory well, but it also provided a historical overview of its development too. This has meant that I’ve learned a little more about several important philosophical figures that I have on my list of people to discover in more depth – Grotius, Pufendorf, Spinoza, and, especially, Leibniz.


Author: Steve C

I work in normative ethics, specialising in animal and ethics and political philosophy.

13 thoughts on “Natural Law Theory and Gay Marriage

  1. Hi great post you made some great points and raised important questions. It’s intellectually generous of you to explore a topic most gay rights activists would scoff at the first mention of and categorically dismiss. I’ve been interested in learning more about natural law theory myself and this guy’s blog is pretty interesting.
    He’s a big fan of Edward Feser. I’m not sure if I buy everything he says but I think reading it’s worth checking out and he’s really great about responding.

    • Thanks for the comment and the link Mike. Coming back to this post made me realise I’d labelled it ‘gay marriage’ rather than ‘equal marriage’ – black mark against me!

  2. Steve, thanks for this post and for my delinquent comment. One of the issues that I am struggling with regarding natural law theory is how it would see the use of technology. Is it a product of our reason and scientific thinking that helps us to explore more deeply the truths found in nature? Or is it an artificial property that, in and of itself, alienates us from natural truths? Or does it do both?

    I ask that fundamental question to reach a more relevant conclusion to your article. If we find, through science and technology, that there is an inherent and demonstrable biological nature in homosexuality, would this change our understanding of sexual relations? And if it does, can we reconcile the procreation principle with this principle as well. Can there be a multi-faceted nature in our sexuality? And if so, does this imply that nature is competent at balancing human determinism itself (e.g., population increase by birth, in-vitro fertilization, surrogacy and population decrease by death, sterilization, or same-sex relations)?

    Forgive the questions, I am leading down the path–as perhaps you have suggested by distinguishing from Divine Command theory–that natural law theory is not static or fixed, but maybe dynamic. In so being, its properties are constant flux. Could such a view Constitutionally justify the legalization of same-sex marriage as much as other interpretive theories?


    • Thanks for the comment Justin – some very interesting questions. As I understand it (this isn’t my field), Natural Law Theory claims that human nature is to do good and avoid evil. Part of doing good for humans involves preserving and reproducing human life. We do good by pursuing our natural ends. The natural end of sex is reproduction, so sexual acts which are not directed at reproduction are wrong.

      Now, if the natural end of sex is not reproduction, but something else (perhaps it has more than one end [insert innuendo here]) then it would seem to follow that those other ends are also good. So, it may be that evidence shows that there’s a biological purpose played by homosexuality, and that would mean that natural law theorists would indeed have to concede that homosexuality is good.

      However, if God has said that the final end of sex is procreation, then the debate is killed-off before the science can even enter the picture. That appears to be the case here – the purpose of sex is a matter of doctrine not discovery: the word of God is always going to be a more authoritative source of knowledge than scientific claims. The use of technology is instrumental to the pursuit of the natural ends, which have already been stipulated in doctrine. Thus, for example, IVF treatment allows us to pursue the natural end of reproduction through technological processes, but scientists can’t tell us anything new about our natural ends.

    • Justin

      Good questions.

      In fact natural law theory, in its classical sense, would be absolutely OK with the use of technology to explore the truths of nature. After all, the pursuit of truth, is considered to be the greatest good for a rational creature to pursue. In fact, his very purpose.

      Regarding biological nature of homosexuality, if this were to be shown it would have no real impact. In fact the scholastics and Aquinas would be perfectly at ease with the idea. As indeed is the Catholic Church today. After all, they regarded concupiscence as innate to individual human natures and in fact inherited by physical descent from Adam. However, they would not conclude that this meant that homosexual acts were moral.

  3. Steve

    An interesting blog. Thank you.

    I am not sure about your reply to Justin. I think you conflate a homosexual act with homosexual inclination itself. Whilst a natural law theorist would be happy to concede that the existence of homosexual inclinations may indicate a biological purpose to the existence of homosexual impulses (why not? After all, purpose goes hand in hand with teleology and natural law is built on teleological ideas) , he certainly would not concede that this meant that such sex acts themselves were necessarily good. He would instead distinguish between the passions of men and the good. Natural law is about using reason to distinguish what is good for man through study of his nature rather than considering the good to simply be the submission to passions (which is hedonism).

    I also cannot agree that the purpose of sex is a matter of doctrine and not discovery. Can we really argue that procreation hasn`t been central to the role of sex in the evolution of homo sapiens? We can no more say the purpose of sex is a matter of doctrine rather than discovery than we can say that supposing the purpose of the heart is to pump blood is as well.

    Regarding your main article. I think you are too fast to say that natural law fails because it commits the so called naturalistic fallacy of deriving ought from is. In fact, this just isn`t so because it is grounded in a metaphysics of final causes and so this fallacy just doesn`t come into play. You will also find that an understanding of the metaphysics, based greatly on the work of Aristotle, would explain why ethical judgments might be made at a category level.

    Also, I think you overstate the case of natural law being adopted by the Catholic church due to the failure of Divine Command Theory. This really isn`t the case. We can already see reference to a form of Natural law right back to St Paul`s letter to the Romans. In other words, from the very beginning of the Church. In fact the popularity of Divine Command Theory in Christianity really came much later with the reformation – i.e. after Natural Law was already well established in Christian philosophy and thought.

    There is plenty more that I could comment on. Natural Law is one of the most misunderstood branches of philosophy, but when studied, is actually one of the most convincing.

    • Thanks Peter. I’m sure I’ve got some things wrong here, so thanks for clarifications. I do want to challenge one of your assertions though. As Justin argued above, it might well be the case that sexual acts serve a biological purpose beyond procreation – that doesn’t sound completely unreasonable a claim to me. Furthermore, if that were found to be the case and it conflicted with some statement in the bible (I’m sure it does), then I really don’t see the Church conceding to the biologists any time soon. As far as the Church is concerned, it isn’t for biologists discover the purpose of our acts, but for God to tell us – this is a case where knowledge is imparted rather than discovered.

      Second: sure, Aristotle’s metaphysics might explain how it is possible to construct an argument that ethical judgements can be made at the category level, but I don’t think that would make such judgements any less unappealing or unconvincing.

  4. Hello Steve

    regarding the point concerning biological purpose.

    I think I was interpreting the phrase “an inherent and demonstrable biological nature in homosexuality” a little differently. I was thinking this was more a reference to a purpose associated with the existence of homosexuality as a tendency or inclination in humanity in general. And as I explained, this would not be an issue.

    However, you are referring to the possibility that sexual acts may have a purpose inherent to themselves beyond procreation and that this may be revealed by biology. Of course, it’s difficult to discuss without identifying what this hypothetical might be, but I would be surprised if it changed anything. After all don’t forget, Aquinas (and the Church) already do believe that there is a purpose to sex besides the procreative, namely the unitive. Therefore the idea that it is more than just procreative is already a very old one. But the issue in natural law is whether a person sets out to behave in such a way that is in opposition to the natural ends (i.e. the good) to a thing. As homosexual sex can never be reconciled with procreation, and as it is undeniable (and science makes it even more so) that a primary natural end of sex is procreation, then it is difficult to foresee how any advance in biology could reconcile it to natural law.

    As to your comment that God tells us the purpose of things and not biologists etc; I don’t think it is so simple – even if it is clear that the Church does believe that some things have been imparted through revelation. Natural Law is based on the idea that we can discern the purpose of things through reason. And science can help with this. And the Church agrees. So, if you like, the Church says that God reveals things through the work of scientists. It does not regard there to be an inherent dichotomy.

    Regarding your second issue; frankly I don’t really know what you have in mind by unappealing /unconvincing. If an argument can be constructed that allows such ethical judgements to be made, then this surely already goes some way to make such judgements more appealing and convincing?

    If we believe, as most people do, that there is purpose to reality, or a teleology if you prefer, then it is not unreasonable to suggest that things with different natures may have different purposes. And I don’t know anybody that does dispute (in practice) that this is so. Therefore if the good is acting in accordance with natural purposes then it is clear that you might treat different categories of things differently.

    In fact, it is when we don’t that we start to get into difficulty. For example, you compare an anencephalic infant with a dolphin. The problem is that you could also have compared a perfectly normal new born infant with a dog and you would still be left with the same question. Yet I doubt that many would value the dog higher despite the fact that it has higher actual cognitive ability than the infant.

    However, a Thomist would look at the nature of things and say that all infants have a rational nature even if they do not display them in the present and may never do so. As soon as you start looking at individual cases on a more utilitarian level, then we open the door to all manner of problems – we can certainly wave good bye to the equality of man.

    None the less, this doesn’t mean that we shut the door on discussing whether we have rightly understood the moral worth of dolphins. And I think most natural law thinkers would say that the more science tells us about them, the more we can reappraise the ethics of our treatment of them. And in fact, this is something that I find particularly appealing in natural law, i.e. the way it can feed off science and yet avoid the “ought” / “is” debate.

    • I’m not sure most people do believe that there is a purpose to reality, I know I don’t. Most people could be wrong in this regard in any case. I guess it depends what you mean by purpose – if it’s a claim that our biology gives us impulses and contributes to what goes well or badly for us, then I’ll concede that. But if you mean it in the sense that we’ve been given a purpose by higher power, or that it’s morally right to act according to our biology (or according to the instruction of a superior power) then I think that’s just nonsense.

      I don’t myself have any difficulty saying that the moral standing of an anencephalic infant is less than that of a dolphin. Comparing a healthy newborn infant with a dog is more difficult, but it may be that our intuitions are simply wrong on this – I suspect that they probably are.

      One thing’s for sure – you’ve encouraged me to get round to Aristotle’s metaethics once I’ve finished reading his ethics.

  5. Hi Steve

    If you don’t think most people believe there is a purpose to reality then try asking people what a heart is for? Or an eye? I think you will in fact find that they will give you answers involving pumping blood or seeing rather than tell you that actually they have no purpose in reality. In other words they do believe that there is purpose to reality. Sure, if you ask a philosophy graduate enamoured by materialist reductionism, they may argue that this is only apparent purpose. But if you actually get to know them, you will find that even they cannot avoid living and talking as though there really is purpose. So we simply can`t dismiss it as nonsense, seeing as none of us live without treating it as real in practice.

    Anyway, I`m pleased if I`ve encouraged you to read some Aristotle. Even if his science didn`t hold up well over the years, his philosophy is actually more relevant than ever. I note your comment about purpose being given by a higher power but when you do read Aristotle you will find that he does not make that claim. Instead, he claims that teleology (purpose) is immanent to the natural order rather than imposed from outside by a higher power. Of course this Aristolean metaphysics will have an impact on natural theology (and Aquinas brilliantly expands on this) but it is wrong to think that natural law itself rests on natural theology.

    As for the dog and baby, the point is to see if we are intellectually coherent with our ethics in general. And if we do this, and decide that our intuition is wrong and that indeed the dog does have greater moral worth according to a given philosophy that also ranks the dolphin higher than the anencephalic , we can then move on to comparing the poor against the rich and so forth. I don`t think that takes us anywhere good. Personally, I find that animal welfare activism has been too ready to embrace this style, aka Peter Singer and co. indeed, I think natural law is actually a more promising way forward towards promoting animal welfare. Whilst it`s understandable that on reading Aquinas and Aristotle we don`t find them too sympathetic to animals we have to remember that they lived in a society where human life was fairly brutal as well. Today however, arguments revolving around purpose and natural flourishing – inherent to a natural law approach, can be constructed using that would find far more sympathy among a wider audience.

    • First, there’s a very big difference between claiming a heart has a function, or that living organisms have biological drives, tendencies, directions of growth etc, and claiming reality has a purpose. There’s no good reason to believe the latter claim.

      Second, for the record, I’ve read the Nicomachean Ethics, a chunk of the Eudemian Ethics, his Politics, his History of Animals, and some of his logic, so I’m reasonably conversant with his claims. Also, I may not have read the original texts of his works on Metaphysics, but I’m not altogether ignorant of the content. I responded to what I thought was claim you were making about the nature of the universe, rather than thinking I was discussing what Aristotle’s claims were.

      Lastly, in your last paragraph you imply that a claim about the moral standing of non-human animals leads to: ‘comparing the poor to the rich’ This is the point where you went from interesting discussion to fallacy and rhetoric. Your final point about placing Aristotle in historical context does you no good either when you have the likes of Pythagoras, Porphyry, Empedocles, Xenocrates, and many other classical figures promoting vegetarianism (albeit for some sometimes quite odd reasons). Even Aristotle’s successor Theophrastus, whom Aristotle left his writings too, advocated vegetarianism. Yes, an Aristotelian inspired approach to virtues can add richness to the debate about how we should relate to non-human animals, but Aristotle on his own, like virtue ethics more broadly, is far from sufficient.

      • Steve

        How can we even discuss whether something is functioning or not unless we implicitly accept some purpose associated with it? Unless the purpose of the heart is to pump blood then we just cannot say it is not functioning when it is not pumping. We can only say that it is not pumping and we certainly can’t then go on to talk about good hearts or bad hearts.

        And of course if there is no reason to believe in a teleology then we can take this right to where Hume left it and say that there is also no reason to believe in causation either.

        As for the study of Aristotle, my only point there was that classical natural law theory is in itself not based on theological claims. If I misunderstood your earlier reference to a higher power, then I apologize.

        And I was not implying that a claim about the moral standing of non-human animals necessarily leads to: ‘comparing the poor to the rich’. Of course this would be untrue. The point I was making was whether the particular reasoning behind a conclusion that a dolphin has more moral worth than an anencephalic infant might lead us to reappraise our views in other instances. You seemed to have accepted that this might be possible when you mentioned that our intuitions may be wrong on the comparison of an infant with a dog. It is therefore not unreasonable (and far from simple fallacy and rhetoric) to suggest that we might go one step further and consider the case between different members of the same species and question whether we are being consistent there as well. This is what prompted my mention of Peter Singer. Although I disagree with him, I can at least recognize when he is being intellectually honest.

        Finally your counterpoint about Aristotle in historical context. I can concede this to some degree – perhaps you are right to suggest that his classical environment itself may not have affected his fundamental views. But I am pleased we can agree that maybe he, and natural law, may have something to offer in this regard.

        By the way, check out this link:

        It has a nice mention of natural law, ethics, and the study of primates.

  6. Getting married in our country is about politics. Two people and the state. If Christians were being obedient to their beliefs, they would enter into Holy Matrimony minus the state (sorry, no tax breaks), not marriage to the state. That is just one reason why I support free will gay marriage. Maybe marriage should be illegal for Christians?

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