Aquinas on What’s Natural and Unnatural

September 16, 2015

Latest Thinking, Natural Law, Natural Rights / Rights, Philosophy of Rights, Private Property Rights

How can death be both natural and unnatural at the same time, in Aquinas’s thought. This post takes up that question.

[This post continues my discussion of why Aquinas holds there was no private property in Paradise. Other topics in the series include in order: 1. No property in Paradise, 2. Life in Paradise, 3. No Labor in Paradise,  4. Sexuality and Procreation in Paradise, 5. The Children of Adam and Eve, 6. Were Adam and Eve Vegetarians?  7. No Equality in Paradise, 8. Subjection and Government in Paradise 9. Private Property, the Result of Sin, 10. Aquinas’s Analogy: Clothing, Slavery and Private Property. Or download the full essay here: No Property in Paradise: How Aquinas Understands the Origin of Private Property]

There is something very confusing, if not a bit slippery, about Aquinas’s conception of property and natural law that is now time to address more directly. The confusion arises because Aquinas assumes that private property does not contradict natural law, according to which all things are common. But why not? Why does the privatization of things not represent a perversion of nature and God’s will instead of a good and reasonable human institution, as Aquinas claims? For me, Aquinas’s answer is less than satisfying. And I am not talking about my own views of property, but how, a deep and conscious thinker like Aquinas, can hold such a position that seems so contradictory.

The question that naturally arises is how does Aquinas distinguishes a human practice that is a perversion of natural law from one that is a reasoned and good extension of it? Why, for example, does Aquinas argue that homosexuality is a perversion of nature but private property is not? How does Aquinas determine that the reversal of what he holds to be a norm in nature is a perversion in one case (homosexuality) and a positive institution in others (e.g. , private property, slavery, clothing).  There are in fact two separate but related questions here. The first is, “By what criterion does Aquinas determine what is natural, and second, by what criterion does he decide that its opposite is a positive human transformation of nature rather than a perversion of it.

One might reasonably argue that Aquinas was a product of his day and therefore his thinking about nature was shaped by the assumptions of his time and the traditions he received. And there would be much to recommend this kind of answer, since the very concept of nature is itself shaped by and in turn shapes a much larger set of assumptions about how and why the world is the way it is. And this point would be a worthy and important one. For it is important to realize that the very concept of what’s natural is itself constructed in each time and place, and that the concept is itself constituted by a whole host of other assumptions and traditions that shape how one draws the lines between what is natural versus what is human. Arguably, it is the line between nature and non-nature that has changed very dramatically in human imagination in the last couple of hundred years.

And yet one still wants to try to enter into the particular versions of this distinction between nature and non-nature and in particular with talented thinkers such as Aquinas who certainly thought that the distinctions they made between what’s natural and what isn’t were meaningful, intelligent and consistent with what God wanted.  Let us turn to this discussion now to see if we can make more sense of Aquinas’s view that private property is a positive development rather than a perversion of nature.

Two ways of being natural

Part of the problem in determining what is natural for Aquinas comes from the fact that the saint  identifies several different ways of talking about what’s natural.  As a result, the question of what’s natural may itself be in part a linguistic problem of how people use language. For example, in discussing the relationship of virtue to the natural law, Aquinas notes that there are different ways of talking about what is natural to human beings or what we call “human nature.”

Reply to Objection 2: By human nature we may mean either that which is proper to man—and in this sense all sins, as being against reason, are also against nature, as Damascene states (De Fide Orth. ii, 30): or we may mean that nature which is common to man and other animals; and in this sense, certain special sins are said to be against nature; thus contrary to sexual intercourse, which is natural to all animals, is unisexual lust [e.g., homosexuality in our language], which has received the special name of the unnatural crime.[1]ST I-II, 94, iii, ad. 2.  The context of this quote is whether all acts of virtue are prescribed by the natural law.

Aquinas tells us that when we speak of human nature, or the nature of humans, we have a bit of a linguistic difficulty since we may be invoking two different ways of talking and referring to two different dimensions of human nature.

Sometimes we may be referring to what is distinctive and unique about humans, which has to do with human reason, which distinguishes them from the other creatures.  What is natural in this sense is in line with reason. Anything against reason is against nature and is a sin.   At other times, we may be talking about the characteristics that humans share with other creatures, which includes bodily functions such as hunger, thirst, procreation, aging and death. [2]For the moment, let’s set aside Aquinas comment that homosexuality is unnatural and the fact that some animals do engage in non-heterosexual forms of sexuality. That is certainly a conversation one … Continue reading

To summarize, then, essentially Aquinas is saying that there are two ways of talking about what’s natural. One way has to do with what makes humans distinctive and has to do with their very purpose or end and their distinctive use of reason. The other has to do with what they share in common with other living beings. Is this simply a linguistic problem? Not completely. Aquinas holds that both ways of talking are acceptable because humans are in fact a type of creature that has two sorts of natures that have been combined together and are in some sense in tension with one another. This mixture of natures is what in fact makes humans unique. And yet the dual nature of the human being leads to some linguistic challenges. Thus he writes:

Reply to Objection 1: A thing is said to be natural if it proceeds from the principles of nature. Now the essential principles of nature are form and matter. The form of man is his rational soul, which is, of itself, immortal: wherefore death is not natural to man on the part of his form. The matter of man is a body such as is composed of contraries, of which corruptibility is a necessary consequence, and in this respect death is natural to man.[3]ST II-II 164, i, ad. 1

Humans are different from both angels and animals but have characteristics of each. Like angels, they have rational souls, which are immortal. But like animals, they have bodies which are corruptible and thus animals can age and die, need food and engage in reproduction. This is why Aquinas can both assert that death is “not natural” when speaking about the human form but is “natural” when we are speaking about the human body. Death is both natural and not-natural at the same time, depending if one is looking at the human from the perspective of the soul or the body, form or matter.

Thus when we speak about something that is unnatural, it can be unnatural in respect to the soul or the body. And this corruption [of nature] may be either on the part of the body—from some ailment; thus to a man suffering from fever, sweet things seem bitter, and vice versa— or from an evil temperament; thus some take pleasure in eating earth and coals and the like; or on the part of the soul; thus from custom some take pleasure in cannibalism or in the unnatural intercourse of man and beast, or other such things, which are not in accord with human nature. [4]ST I-II 31, vii

Human nature is thus characterized by what Aquinas calls “contraries” or contradictions. This dual nature of the human being is partially responsible for the difficulty of following what Aquinas and others mean linguistically by what’s natural and unnatural. There are natural dimensions of both soul and body and each can have its unnatural counterparts. One can thus categorize natural and unnatural as a matrix intersecting with Reason (Form/ Soul) and Bodies (Matter), as follows:

Soul (Reason / Distinctive to Humans)Body (Humans Share with Animals)
Unnatural Death


As we can see from this tabular representation, death can be both natural and unnatural depending on which dimension of the human being we are speaking about. Death appears as natural when it is considered in common with animals. But death is unnatural when it is considered in relationship to the soul. To fill in the matrix more with other natural and unnatural practices that Aquinas mentions, it would look like this:

Soul (Reason / Distinctive to Humans)Body (Humans Share with Animals)
NaturalHuman law, virtue, government, Adoration and contemplation of GodSexuality, Thirst, Hunger Aging, Procreation (Heterosexual), Death
Unnatural Cannibalism, Bestiality, Sin, DeathFever, Illnesses of the body, Homosexuality


Aquinas makes a similar kind of distinction when explaining the origin and types of human concupiscence (strong desires and pleasures). Here he distinguishes desires that arise from the creatureliness of the human being versus from the soul.  However, in this case, instead of concluding both are “natural,” he calls natural only those that humans share with creatures. By contrast, those that are unique to humans are “unnatural.” Here is Aquinas again:

Now a thing is pleasurable in two ways. First, because it is suitable to the nature of the animal; for example, food, drink, and the like: and concupiscence of such pleasurable things is said to be natural. Secondly, a thing is pleasurable because it is apprehended as suitable to the animal: as when one apprehends something as good and suitable, and consequently takes pleasure in it: and concupiscence of such pleasurable things is said to be not natural, and is more wont to be called “cupidity.”

Accordingly concupiscences of the first kind, or natural concupiscences, are common to men and other animals: because to both is there something suitable and pleasurable according to nature: and in these all men agree; wherefore the Philosopher (Ethic. iii, 11) calls them “common” and “necessary.” But concupiscences of the second kind are proper to men, to whom it is proper to devise something as good and suitable, beyond that which nature requires. Hence the Philosopher says (Rhet. i, 11) that the former concupiscences are “irrational,” but the latter, “rational.” And because different men reason differently, therefore the latter are also called (Ethic. iii, 11) “peculiar and acquired,” i.e. in addition to those that are natural. [5]ST I-II, 30 iii 2852

We can again represent this position again in tabular form.


(Reason / Distinctive to Humans)


(Humans Share with Animals)

Natural ConcupiscencesFood, Drink, called “Common and Necessary” and “Irrational”
Not Natural ConcupicensesDevising something as good and suitable beyond what nature requires. Tend to be called “Cupidity.” Are called “Rational.” Also “peculiar and acquired.”


We see here that the concupicenses ( strong desires) that are distinctive to humans, and are rationally based, are also called “not natural,” whereas when talking about pleasures, as we saw above, what was distinctive to the human was also called “natural.”  Thus, we have some things that are distinctive to humans which are “natural” and some which are “ not natural.” Furthermore, Aquinas following Aristotle calls the natural pleasures which humans and animals share “irrational.” He thus associates the pairs “natural” and “irrational,” on the one hand, and “unnatural” and “rational,” on the other.  This seems linguistically quite odd since Aquinas thinks of nature and natural law as a rational, organized hierarchy in general. As we shall now see, matters get still more complicated because Aquinas also refers to some natural qualities of the body as “defects”

Next Up:  On Natural Defects and Their Removal in Paradise



1ST I-II, 94, iii, ad. 2.  The context of this quote is whether all acts of virtue are prescribed by the natural law.
2For the moment, let’s set aside Aquinas comment that homosexuality is unnatural and the fact that some animals do engage in non-heterosexual forms of sexuality. That is certainly a conversation one wishes one could have with Aquinas and discuss with him instances such as the Bnobo who have homosexual and nonreproductive sexuality. One can imagine he might say that they are corruptions of nature, but he would certainly have to explain why a species seems to naturally have homosexual sexuality.
3ST II-II 164, i, ad. 1
4ST I-II 31, vii
5ST I-II, 30 iii 2852