Tradition — Christianity
essays
Aquinas, or How Nature Became a Verdict
Thomas Aquinas, Aristotelian *telos*, and the grammar of natural / unnatural that still wires doctrine to law
Historical argument
What Thomas Aquinas actually argued about sex and natural law, how Aristotle reshaped Latin Christianity, what earlier Christian idioms his synthesis sidelined, and why “natural / unnatural” still echoes in magisterial teaching and US legal argument.
It is possible to read the history of Christian sexual ethics as a series of answers to a single uncomfortable question: who gets to say what the body is for? The answers have never been one voice. They have been a quarrel — sometimes muted, sometimes explosive — among Scripture, ascetic heroism, Roman legal habits, Greek philosophy, clerical bureaucracy, motherhood mysticism, monastic terror of scandal, and the plain human need to feel that desire is not an insult to dignity.
If pillar one of this arc traced Augustine — how shame, concupiscence, and a particular reading of Genesis became Latin Christianity’s default grief about the body — then this pillar traces the man who did not undo that grief but ran rational cable through the walls: Thomas Aquinas, Dominican friar, Parisian master, author of the Summa Theologiae, dead in 1274 at forty-nine or fifty, leaving behind a system so architecturally complete that much of what Catholics and many Protestants still mean by “natural law” is really Thomistic wiring installed after Augustine’s foundations were already poured.
The case here is not that Aquinas invented sexual guilt, any more than Augustine invented it. It is that Aquinas gave Western Christianity a vocabulary in which “natural” and “unnatural” function as moral binaries backed by metaphysics — a vocabulary elastic enough to travel from thirteenth-century confession manuals to twenty-first-century Supreme Court dissents that still reach for “ordered” and “disordered,” telos translated into “conjugal.” What Aquinas systematized was not only a list of forbidden acts. It was a way of seeing: the human person as matter organized toward rational ends, the sexual faculty as ordered to procreation and marital fidelity, pleasure as innocent when folded inside that order and vicious when it slips the fold.
The record also shows what that system displaced — not by wiping it from the manuscripts, but by making an alternative imagination harder to think. Before Aquinas, Latin Christianity had already absorbed Augustinian concupiscence and a patchwork of penitential severity; what it did not yet have was a fully elaborated Aristotelian account of human nature underwriting every moral verdict. Greek theology, meanwhile incomplete in Aquinas’s own training, had long spoken the language of theōsis, of economia, of marriage blessed without the same deductive bridge from species to act. Aquinas’s synthesis is the moment the West learns to argue about sex the way it argues about birds and ships: by species, function, accident, and end.
This is pillar two of four. Augustine named the wound. Aquinas built the clinic.
I. The work on the desk
Thomas was born in 1225 into minor nobility between Rome and Naples, sent as a boy to the Benedictine monastery at Monte Cassino, then to the University of Naples, where he encountered the new Aristotelian translations and the restless poverty of the Dominican friars. His family kidnapped him to stop his joining; Thomas insisted. By 1245 he was in Paris studying under Albertus Magnus; by the 1250s he was composing the works that would make him, in the Catholic imagination, the theologian — the Summa contra Gentiles, then the unfinished Summa Theologiae, commentaries on Aristotle, treatises On Evil, On Truth, On the Power of God.
Nothing in that biography predicts prurience. Aquinas is not Augustine; he does not write spiritual autobiography about beds he has left. What he writes is analytical prose — questions, objections, answers, replies — in which the human animal is inspected with the same steady patience Aquinas brings to angels and sacraments. The point matters because the modern reader who expects psychological heat from Aquinas will feel cold. The heat is structural. The drama is classification.
Aquinas inherits Augustine’s basic Augustinian knots: fallen desire, necessity of grace, marriage’s threefold good, the suspicion that sexual pleasure unbundled from procreative intent carries the stain of concupiscentia. What Aquinas adds is Aristotle’s Physics and Nicomachean Ethics read through Neoplatonic commentary: nature as inner principle of motion and rest; every substance as oriented toward a form and a final cause; virtue as the habit that aligns the rational creature with its telos. For Aquinas, “natural law is nothing else than the rational creature’s participation in the eternal law.”[^1] That single sentence — participatio — is the hinge on which centuries of moral argument swing.
So when Aquinas turns to sex, he is not only repeating Augustine’s pastoral anxieties. He is asking an Aristotelian question: what is the sexual power for? The answer he gives — in outline before the details — is that the sensitive appetite’s generative act is ordered per se toward the good of offspring and the good of the union of spouses; variance from that order is preter intentionem naturae — beside the intention of nature — and receives moral names Aquinas thinks follow necessarily: peccatum, vice, disorder.
Modern readers often hear “natural” as “normal” or “found in animals.” Aquinas means something tighter: natural is what belongs to the rational animal’s integral flourishing as such. The squirrel does not deliberate virtue; comparing human sex to squirrel sex is category error in Thomistic terms. The medieval list of “against nature” acts is not pretending to be ethnography.
II. The species of lust
Aquinas treats sexual morality primarily under the virtue of temperance — the moderation of touch and taste — and specifically in the secunda secundae, the second part of the second part of the Summa, questions on chastity and the vice opposite it, luxuria.
Chastity, for Aquinas, “narrows” desire so that it does not wander beyond right reason.[^2] Right reason here includes both the bonum matrimonii and the order of creation itself. Marriage is natural for the human species in a way celibacy is not — Aquinas is explicit that virginity exceeds the precept of nature and belongs to a counsel of perfection — but the sexual act within marriage still must answer to the same teleology.[^3]
The vices Aquinas catalogues look, to a modern eye, like a taxonomy from a penitential handbook translated into philosophy. Fornication, adultery, seduction, rape (which he names among the species of injustice, not merely lust), incest, sacrilege (sex violating sacred vows or places), unnatural vice, masturbation framed as incomplete or disordered emission — each receives a species name and a causal account tying injury to God, neighbor, or self according to the object chosen.
What distinguishes Aquinas from a compiler of prohibitions is the insistence that these are not arbitrary divine whims. They follow, he argues, from the structure of the good the act ought to serve. When pleasure is pursued “not in accord with right reason” — non secundum rectam rationem — the will chooses a disordered end.[^4]
Unnatural vice among the species of lust (*ST* II-II, q.154, a.12)
Primary source
“Vices against nature are also against God, as stated above (Reply to Objection 1), and are so much more grievous than the depravity of sacrilege, as the order impressed on human nature is prior to and more firm than any subsequently established order.”
Thomas Aquinas — *ST* II-II, q.154, a.12, Reply to Objection 2 (English Dominican Province trans., 1920). The *body* of the article argues unnatural vice is gravest *among species of lust*; Reply Obj. 4 subdivides which unnatural acts are worse than which.
Modern reading
MacCulloch reads high scholasticism as turning penitential lists into a metaphysical grid — "natural" and "unnatural" gaining Aristotelian armor that pastoral handbooks alone had not supplied.
Diarmaid MacCulloch, *Lower than the Angels: A History of Sex and Christianity* (Viking, 2024/2025) — scholastic consolidation.
Counter-argument
Medievalists emphasize that "sodomy" in canon law was a shifting bundle of acts and social panics; mapping modern sexual identity onto Aquinas’s *species peccati* frame risks anachronism.
Mark D. Jordan, *The Invention of Sodomy in Christian Theology* (Chicago, 1997); Ruth Mazo Karras, *Sexuality in Medieval Europe* — category history vs. act-theory.
Editorial documentation of sources — not a legal or ecclesiastical brief.
III. Unnatural vice as the purest case
The most consequential stretch of Aquinas for contemporary fights — ecclesial, cultural, legal — runs through Summa Theologiae II-II, question 154, especially articles 11–12 on vice contra naturam. Article 11 asks whether unnatural vice is a species of lust at all; article 12 asks whether it is the gravest such species. The body of article 12 argues that because reason presupposes what nature fixes, acting against natural determinations in the use of sex is, in this genus, the gravest corruption.[^5]
The ordering readers usually cite — bestiality, sodomy, then other “right species, wrong manner” abuses — appears where Aquinas subdivides sins against nature in the reply to the fourth objection: the worst is bestiality (wrong species), then sodomy when “use of the right sex is not observed,” then failures of due manner. The language is not modern; the logic is binary in a technical sense: either the act-type answers to the procreative and marital order Aquinas attributes to human nature or it does not.
There is no Thomistic category for morally good non-procreative spousal sex aimed only at union — that ressourcement belongs to twentieth-century revision, not the thirteenth-century text. For Aquinas, if you knowingly choose pleasure while positively excluding the procreative meaning — or choose an act-type whose structure contradicts it — you have chosen contra naturam in the moral sense he defines.
Two clarifications matter for fair reading.
First, Aquinas is not offering empirical predictions about children’s welfare in same-sex households; such sociology does not exist in his tool kit. His argument is metaphysical: the structure of the act relative to the potency’s telos.
Second, Aquinas does distinguish severity and circumstance. He discusses ignorance, passion, habit — the ordinary apparatus of moral theology. What he does not do is provide a vocabulary for orientation as interior identity. “Homosexuality” as modern taxon is absent. The analysis is of chosen acts under species — which is why contemporary defenders and critics of Thomistic sexual ethics argue past each other: one side reads timeless metaphysics; the other reads silences that later discourse filled differently.
Downstream, these articles feed Gratian’s Decretum, later manuals for confessors, the confessional grid of early modern Catholicism, and — only after many mediations — the 1917 and 1983 codes of canon law and the Catechism’s still-Thomistic skeleton on “intrinsically disordered” acts.[^6] The chain is not a straight pipe — Trent, probabilism, casuistry, and Vatican II all complicate it — but the grammar of genus and rational teleology is recognizably Aquinas’s.
IV. What synthesis replaced
Aquinas did not invent Christian suspicion of non-procreative sex. He did not invent clerical celibacy, penitential tariffs, or the equation of pleasure with danger. What he did was bring under one roof what had been, in the Latin West, a noisy basement of partial theories.
Before the high Middle Ages, Greek patristic writers — John Chrysostom, Basil, Gregory of Nyssa — often preached ascetic preference for virginity while blessing marriage as real vocation. Their rhetoric could be ferocious about theatrical female adornment and male self-indulgence, but their cosmology was less a single deductive system than a liturgical-pastoral weave. Eastern canon law could permit economies of remarriage under penance while still praising monastic ideals; the gap between ideal and practice was negotiated through economia, not always through species-level act analysis.
Augustine had already tied original sin to sexual transmission in a way the East resisted. Aquinas accepts Augustine’s basic anthropology but refines it with Aristotelian hylomorphism: body-soul composite, powers flowing from essence. He keeps Augustine’s unease about concupiscentia while giving it a slot in a virtue theory where temperance is one of several hinges.
The figure Aquinas most thoroughly does not sound like is the casual moralist who says “natural” because culture applauds. His “nature” is recursive — it passes through eternal law, natural law, human law, and divine positive law in the famous Summa articles on law in I-II 90–97.[^7] That architecture is what allowed later Catholic natural-law thinkers — and, selectively, Protestant scholastics — to claim that sexual norms were not sectarian taboos but conclusions any reasonable person could see, given the obvious ends of human bodily life.
Natural law’s first precepts (I-II, q.94, a.2)
Primary source
“such as sexual intercourse, education of offspring and so forth”
Thomas Aquinas — *ST* I-II, q.94, a.2, *I answer that* — among inclinations man has in common with other animals, those things “which nature has taught to all animals” belong to the natural law (English Dominican Province trans., 1920).
Modern reading
MacCulloch contrasts the new scholastic unity of "nature" and morals with the messier patchwork of early medieval monastic and penitential practice — Aquinas did not invent severity but rationalized its vocabulary.
Diarmaid MacCulloch, *Lower than the Angels* — Latin West before and after the *via antiqua* synthesis.
Counter-argument
Orthodox personalist theology, especially in Zizioulas’s wake, stresses *hypostasis* and *communion* before essentialist *nature* — a moral lexicon some see as less binary on sexual anthropology than high Thomism.
John D. Zizioulas, *Being as Communion* (St Vladimir’s Seminary Press, 1985) — person and nature in Greek patristic idiom; contrast case to Latin *natura* as moral first principle.
Editorial documentation of sources — not a legal or ecclesiastical brief.
The pre-Thomistic Latin world still knew voices that treated marriage as remedy for weakness without making every non-procreative dimension a metaphysical failure mode — Ambrose’s sternness lives beside warmer strands in Leo the Great’s nuptial theology, themselves unequally distributed. Aquinas reads selectively; everyone does. His selection privileges Aristotle and Augustine together — a pairing neither Greek East nor early Syriac Christianity had fully standardized.
V. Marriage, sacrament, survival of the Augustinian skeleton
On marriage, Aquinas follows Augustine’s triad — fides, proles, sacramentum — but he reorganizes it through sacramental theology and the new thirteenth-century consensus that marriage between baptized persons is itself sacrament of Christ–Church union.[^8] Offspring and fidelity remain goods; the sacramental bond elevates the natural institution.
Aquinas still treats sexual pleasure in marriage more generously than many popular Augustinian caricatures: pleasure is not sin when annexed to a virtuous act ordered to the marital good — it is spiritual matter, and can even increase merit when the intention is upright.[^9] The ceiling of that generosity, though, remains teleological. Pleasure pursued with deliberate exclusion of procreative order — or through acts Aquinas classifies as intrinsically disordered — does not become licit because spouses love each other.
This is the bridge to the textbook Catholic teaching a secular reader meets in fights about contraception: Humanae Vitae (1968), whether one accepts or rejects it, is Thomistic in skeletal form — the inseparability of unitive and procreative meanings as taught by the magisterium is an attempt to update Aquinas’s act-species analysis for technological society. Later personalist currents (Wojtyła) add interiority Thomistic texts underemphasize, but the veto on chosen act-types labeled contra naturam stays recognizably medieval.
Marriage goods and the Eastern contrast
Primary source
“Matrimony is instituted both as an office of nature and as a sacrament of the Church. As an office of nature it is directed by two things, like every other virtuous act… one of these is required on the part of the agent and is the intention of the due end, and thus the “offspring” is accounted a good of matrimony; the other is required on the part of the act, which is good generically through being about a due matter; and thus we have “faith,” whereby a man has intercourse with his wife and with no other woman.”
Thomas Aquinas — *ST* III, *Supplement*, q.49, a.2, *I answer that* — Augustine’s three goods in scholastic idiom (English Dominican Province trans., 1920).
Modern reading
Fredriksen tracks how Latin original-sin discourse tied guilt and disordered desire to procreation in ways Eastern *ancestral sin* idioms never quite mirrored — the backdrop against which Thomistic teleology lands.
Paula Fredriksen, *Sin: The Early History of an Idea* (Princeton University Press, 2012).
Counter-argument
Feminist and womanist moral theologians critique reducing persons — especially women — to generative *telos* without remainder, from within Catholic conversation.
Lisa Sowle Cahill, *Sex, Gender, and Christian Ethics* (Cambridge, 1996); broader ressourcement versus neo-scholastic sexual manuals.
Editorial documentation of sources — not a legal or ecclesiastical brief.
VI. The migration: from Summa to statute
Aquinas’s natural law is not only confessional. Secularized descendants appear wherever Anglo-American jurisprudence borrows “ordered liberty,” “moral truth,” or “deeply rooted” traditions to evaluate legislation about sex, reproduction, and marriage. This is not a conspiracy; it is a line of intellectual descent — Suárez, Grotius-with-Thomistic residues, Blackstone’s marriage law, nineteenth-century common-law marriage theory, twentieth-century Catholic philosophers entering academic law (John Finnis, Germain Grisez, Robert George), and evangelical-Protestant natural-law eclecticism that may never read a page of Latin but still speaks of “complementarity” as if it were obvious.
The dissenting opinion’s moral vocabulary in Lawrence’s aftermath; the echoes in Obergefell dissents about millennia of tradition; the Idaho bathroom bill drafting rooms where “biology is destiny” — these are not crude Thomism, but they share a family resemblance with a world in which telos became public reason.[^10]
Fairness requires the converse: plenty of Thomists reject easy political translations; plenty of secular lawyers reject natural-law metaphysics root and branch. The migration is not fate. It is a sediment: ideas survive their first aquifers.
Human law and the natural-law migration
Primary source
“Now human law is framed for a number of human beings, the majority of whom are not perfect in virtue. Wherefore human laws do not forbid all vices, from which the virtuous abstain, but only the more grievous vices, from which it is possible for the majority to abstain; and chiefly those that are to the hurt of others, without the prohibition of which human society could not be maintained: thus human law prohibits murder, theft and such like.”
Thomas Aquinas — *ST* I-II, q.96, a.2, *I answer that* — why civil law tolerates some private vices while criminalizing others (English Dominican Province trans., 1920).
Modern reading
Finnis’s programmatic jurisprudence re-presented Thomistic practical reason for Anglophone legal theory — a bridge from *ST* I-II 90–97 to rights-talk and constitutional argument.
John Finnis, *Natural Law and Natural Rights* (Clarendon, 1980).
Counter-argument
Legal positivists separate validity of law from moral truth; liberal neutrality asks whether *comprehensive doctrines* of human *telos* should structure public coercion.
H.L.A. Hart, *The Concept of Law* (1961), Ch. IX; John Rawls, *Political Liberalism* (Columbia, 1993) — law, morals, public reason.
Editorial documentation of sources — not a legal or ecclesiastical brief.
VII. Reading Aquinas now
Two truths sit in tension, and both belong in the record.
Aquinas is a mind of astonishing scope — the rare medieval author who can be wrong on particulars and still teach you how arguments hold together. His ethics are not mere clerical control; they are an attempt to show how desire, reason, grace, law, and community cohere in one creature. Anyone who cares about the history of the West’s conscience owes him patient reading, not straw effigies.
Aquinas is also the great systematizer of the binary that still tells millions of queer people that their loves are by metaphysical definition disordered — a verdict pronounced in cool Latin rather than street slurs, but carrying social weight once wired into law and clinic and family. The East never needed this exact verdict structure to produce holiness; the West did not have to embrace it as exhaustive moral truth. That it did has consequences separable from Aquinas’s personal sanctity or intellectual genius.
The pre-Aquinas Christian world did not speak with one throat, but it often trafficked in economies of mercy, ascetic ideals coexisting with pastoral fudge, Greek economia alongside Latin rigor. Aquinas did not erase those strands from history; he offered a rival centre of gravity: nature read through Aristotle, sin read through species, law read through participation in the eternal. Once that centre held, arguing inside Western Christianity without passing through Thomistic checkpoints became difficult — until, centuries later, some theologians learned to reread Aquinas against parts of Thomism, or to foreground Lyotard’s “pagans” in Christian clothing.
The question this arc keeps open is not whether Aquinas was brilliant. He was. It is whether the specific shape his brilliance gave to “natural / unnatural” must remain the only grammar Christians — or their neighbours touched by Christian legal sediment — can use to speak about sex without shame.
Theology is never only ideas. It is also who gets hurt when ideas wear uniforms. Aquinas wrote in brown habits and lecture halls. The ideas marched elsewhere.