Saint Thomas Aquinas and Astrology

I.Historical Context: The 13th Century and the Reception of Aristotle

EIn the 13th century, Europe was experiencing an unprecedented intellectual revolution. Translations from Greek and Arabic had made the complete Aristotelian corpus available, along with it, the astronomy of Claudius Ptolemy and his Tetrabiblos, the most influential judicial astrology treatise of Antiquity. The universities of Paris, Bologna, Toledo, and Naples were torn between fascination with this recovered knowledge and the pastoral concerns it aroused.

Astrology enjoyed enormous intellectual prestige. It was not a marginal popular knowledge: physicians, astronomers, and clergy practiced it; kings consulted their astrologers before battles; and university medicine included the study of the "critical days" of diseases according to lunar phases. Emperor Frederick II Hohenstaufen, a contemporary of Saint Thomas, kept astrologers at his Sicilian court and had promulgated constitutions (the Constitutiones Augustales, 1231, Melfi) that regulated official astrological practice.

In this context, the Church had to pronounce itself. It was not enough to repeat the generic condemnations of Saint Augustine (who had attacked the deterministic astrology of the Manichaeans): a rigorous analysis was needed to distinguish the licit from the illicit, the natural from the superstitious. This task definitively fell to Saint Thomas Aquinas (1225-1274), a Dominican born in the castle of Roccasecca, trained in Naples, Paris, and Cologne (under Saint Albert the Great), and a professor at the University of Paris.

II.Saint Thomas Aquinas: the Angelic Doctor

Thomas Aquinas is, without a doubt, the most influential theologian of scholasticism. Born in 1225 in the kingdom of Sicily (today Italy), he entered the Order of Preachers against the will of his noble family, who kept him for a year in the castle of Roccasecca to dissuade him. His training included Aristotle, Neoplatonism of Pseudo-Dionysius, the Bible, and the Fathers of the Church, especially Saint Augustine and Saint John Damascene.

His work is vast: the Summa Theologica (unfinished, interrupted by his death in 1274 on his way to the Council of Lyon), the Summa contra Gentiles, the Commentaries on the Sentences of Peter Lombard, the commentaries on Aristotle, disputed questions (De veritate, De malo, De potentia), and treatises such as the De occultis operibus naturae. In all of them, Thomas demonstrates a singular capacity for synthesis: he adopts Aristotelianism without renouncing revealed data, and builds a system in which faith and reason, nature and grace, heaven and earth find their hierarchical place.

The Church canonized him in 1323 and declared him a Doctor of the Church in 1567. His doctrine, taken as obligatory reference by the Council of Trent (1545-1563) and by the Roman Catechism of 1566, is known as Thomism and constitutes the main theological current of preconciliar Catholicism. Leo XIII, in the encyclical Aeterni Patris (1879), recommended him as a model of philosophical-theological formation in the seminaries.

"All that I have written seems to me like straw compared to what has been revealed to me." Thomas, after the beatific vision of December 6, 1273, shortly before dying. Cited by Friar Ráyinaldo of Piperno, his confessor and secretary.

III.Position toward astrology: the fundamental distinction

The most systematic treatment of astrology in Saint Thomas is found in the Summa Theologica, II-II, question 95 ("On divination"), articles 2 to 5. There he establishes the distinction that would become classic in all subsequent Catholic theology:

Thomas admits that celestial bodies exercise a physical influence on the sublunar world. The Sun heats, the Moon governs the tides and biological cycles, the stars determine the seasons. This influence is real, physical, and observable, and therefore can be legitimately studied. Natural astrology is, in essence, what we today call applied astronomy to meteorology, agriculture, and medicine.

«Licet observare stellarum motus ad praesentiendum corporales effectus qui ex eis dependent, ut pluvias et siccitates, et per consequens de ubertate et sterilitate fructuum, et per consequens etiam de quibusdam sanitatis effectibus, quia medici criticalis dies secundum lunae dispositionem observant.»

It is lawful to observe the stars to foresee bodily effects that depend on them, such as rain and drought, and from there the harvests, and consequently to foresee also certain things concerning the health of the body, since it is known that physicians observe critical days according to the phases of the moon.

This natural astrology is taught in medieval universities as part of the quadrivium (arithmetic, geometry, astronomy, music) and of medicine. It is not superstitious because it is based on real physical causes and its predictions are probable, not necessary.

What Saint Thomas firmly condemns is judicial astrology: that which claims to "judge" (hence iudiciaria) and predict with certainty human free acts, particular events, and the destiny of persons from the astral configuration of birth. This claim is sinful because:

«Superstitiosum et illicitum est ut per observationem astrorum cognoscantur futura contingentia vel actus humani, qui non subiciuntur necessitati causarum caelestium.»

It is superstitious and illicit to make use of the observation of the stars to know future contingents or the actions of men, which are not subject to the necessity of celestial causes.

Study of astral influences on material bodies: meteorology, agriculture, tides, biological cycles, critical days in medicine. It is based on observable physical causes and its predictions are probable, not necessary.

Claim to predict with certainty human free acts, particular events, and the destiny of persons. It denies freedom, usurps the divine prerogative, and is superstitious and idolatrous in its origin.

Thomas does not approach astrology through popular practice, but through the most prestigious scientific source of his time: the Tetrabiblos of Claudius Ptolemy (2nd century), translated from Arabic to Latin by Plato of Tivoli in 1138. Ptolemy established a distinction that Thomas would assume and radicalize: "universal" astrology (about kingdoms and general phenomena, meteorological and political) versus "genethlialogical" astrology (the individual birth chart). For Ptolemy, both were legitimate and were based on the physical causality of the heavens.

Thomas's operation consists of accepting Ptolemy's universal-natural part (which fits with licit natural astrology) and rejecting the genethlialogical-determinist part (which clashes with Christian freedom). It is a true sieve: ancient knowledge is not rejected as a whole, but what is compatible with faith is integrated and what is incompatible is discarded. This method of critical assimilation is the mark of scholasticism at its best.

The treatise De occultis operibus naturae ("On the hidden operations of nature"), traditionally attributed to Saint Thomas although its authenticity is debated, develops the theme of "occulta": natural operations whose cause is not evident to the senses. There the astral influence is included among legitimate hidden causes: the Moon on the tides, the Sun on plant growth, certain stones and herbs with properties not explainable by the humoral theory of the time.

The key is that, for Thomas, what is "hidden" is not "magical". A hidden operation is simply a natural operation whose cause we do not see, but which obeys the laws of nature created by God. Natural astrology legitimately studies these hidden operations. What is condemned is attributing them to demonic pacts, to supernatural uncreated forces, or to a determinism that denies freedom.

• Denies human freedom: if the stars determine choices, man is not free, and morality (and salvation) become meaningless.
• Usurps the divine prerogative: only God knows future contingents with certainty; pretending to know them through the stars is a form of pride.
• Is superstitious: attributes to physical causes (the stars) effects that exceed their causal power (the will is of spiritual order, not material).
• Tends toward idolatry: historically, judicial astrology is linked to the worship of stars as gods (Chaldeans, pre-Islamic Arabs).

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««Licet obseruare stellarum motus ad præsentiendum corporales effectus qui ex eis dependent, ut pluuias et siccitates, et per consequens de ubertate et sterilitate fructuum, et per consequens etiam de quibusdam sanitatis effectibus, quia medici criticalis dies secundum lunæ dispositionem obseruant.»»

It is lawful to observe the stars to foresee bodily effects that depend on them, such as rain and drought, and from there the harvests, and consequently to foresee also certain things concerning the health of the body, since it is known that physicians observe critical days according to the phases of the moon.

Summa Theologica II-II, q.95, a.5, ad 3

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««Superstitiosum et illicitum est ut per obseruationem astrorum cognoscantur futura contingentia uel actus humani, qui non subiciuntur necessitati causarum cælestium.»»

It is superstitious and illicit to make use of the observation of the stars to know future contingents or the actions of men, which are not subject to the necessity of celestial causes.

Summa Theologica II-II, q.95, a.5, respondeo

IV.The philosophical argument: Why the stars do not determine the will?

The core of the Thomist argument is an application of the Aristotelian doctrine of causality and the Christian anthropology of the soul. The key points are:

This distinction —inclinatio vs. necessitas— is crucial. Thomas does not deny that there are more choleric temperaments under Mars or more melancholic ones under Saturn (in the sense of a humoral predisposition); what he denies is that this temperament determines moral choices. Man can resist the inclination through grace and virtuous effort.

«Corpora caelestia possunt esse causa dispositionum quae inclinant ad passiones, sed non possunt esse causa voluntatis, quae est potentia animae rationalis; anima autem rationalis non subdicitur corporibus caelestibus, sed est superior eis.»

The celestial bodies can be the cause of dispositions that incline toward passions, but they cannot be the cause of the will, which is a power of the rational soul; and the rational soul is not subject to celestial bodies, but is above them.

• The stars are material bodies and therefore can only act as material causes: they influence other bodies through heat, light, and motion.
• The human body receives this influence: the humors, passions, and temperamental tendencies can be affected by astrological cycles (just as the Moon affects the mentally ill —hence "lunatic"— or the menstrual cycle).
• But the will and intellect are faculties of the rational soul, which is spiritual and immaterial. The spiritual is not subject to the material causality of the heavens.
• Therefore, the stars can <em>incline</em> (as a tendency), but not <em>determine</em> (as a necessity). The will remains free.

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««Corpora cælestia possunt esse causa dispositionum quæ inclinant ad passiones, sed non possunt esse causa uoluntatis, quæ est potentia animæ rationalis; anima autem rationalis non subdicitur corporibus cælestibus, sed est superior eis.»»

The celestial bodies can be the cause of dispositions that incline toward passions, but they cannot be the cause of the will, which is a power of the rational soul; and the rational soul is not subject to celestial bodies, but is above them.

Scriptum super Sententiis, lib. II, dist. 15, q.1, a.3

V.The exception: the "mentally ill" and the brutes

Thomas admits an exception: irrational animals (brutes) are determined by the stars, because they lack reason and free will. Similarly, the severely mentally ill may be more influenced by the stars to the extent that their reason is diminished. This explains, for Thomas, why the Moon affects lunatics: not because the Moon "determines" their soul, but because their reason is already impaired and, therefore, the lunar humoral influence acts without the counterbalance of rational will.

This exception is important because it shows that Thomas does not make a dogmatic denial of astral influence, but a precise delimitation of its scope: the influence exists in the corporeal and the passionate; it does not exist in the spiritual and free.

VI.Reception by the Church: from Thomism to the Catechism of Trent

The Thomistic distinction was received as safe doctrine by the Catholic Church and was incorporated into official teaching. The main milestones are:

The distinction was taught in Catholic seminaries until the Second Vatican Council (1962-1965) and remains as the classic theological position, still included in the Catechism of the Catholic Church (1992), n. 2116, which condemns "all forms of divination", including astrology, insofar as they claim to "foretell the future" (judicial astrology).

• Council of Trent (1545-1563): although it does not nominally condemn astrology, it adopts the Thomistic doctrine on human freedom and divine providence, which is incompatible with astrological determinism.
• Roman Catechism (1566): drafted by order of Pius V (Dominican, trained in Thomism), it reproduces the distinction between natural and judicial astrology when commenting on the first commandment.
• Bull <em>Coeli et terrae</em> of Sixtus V (1586): explicitly condemns judicial astrology, citing Thomistic doctrine as its basis.
• Encyclical <em>Aeterni Patris</em> of Leo XIII (1879): recommends Thomism as the official philosophy of the Church, consolidating the Thomist position as a normative reference.

VII.The Jews in the Summa: the doctrine of the witness and subordination

The Summa Theologica does not deal only with astrology. In the Secunda Secundae, question 10, Thomas Aquinas articulates the canonical doctrine on infidels —and, among them, the Jews— that the pre-conciliar Church would maintain for seven centuries. It is not a marginal note. It is the basis of medieval canon law on religious minorities.

The doctrine is articulated on two pillars. The first, inherited from Augustine (Contra Faustum XII, 13; Enarrationes in Psalmos 59), is the doctrine of the «witness»: the Jews must not be killed. Augustine grounds this in Psalm 59, verse 11: «Ne occideris eos, ne quando obliviscantur legis tuae» — «do not kill them, lest they forget your law». The Jews, dispersed among the nations, are living witnesses of Christian truth: their survival and their book prove that the promise was real and that they, by not recognizing it, fell into blindness. To kill them would be to destroy the testimony. The pre-conciliar Church never abandoned this doctrine: not in the councils, not in the bulls, not in the Catechism of Trent. Before Augustine, John Chrysostom had articulated the harshest position: in his eight homilies Adversus Iudaeos (c. 386-387), he calls the synagogue a «den of thieves and lair of wild beasts» and forbids Christians to participate in their feasts. The Augustinian doctrine of the witness tempered this severity without abandoning the theological condemnation.

The second pillar is subordination. Thomas articulates it in the Summa II-II, q.10, a.8, ad 2um. The objection poses: if the Jews cannot be forced to baptism (because faith requires will), should they not at least be deprived of their civil rights and reduced to servitude? Thomas answers by citing the Augustinian doctrine and adds the theological reason: the Jews, who received Christ in the Passion, incurred the guilt of deicide, and for that guilt they are destined to perpetual servitude.

The phrase is deliberately harsh and Thomas does not soften it. The servitude is not racial —the racial category does not exist in the 13th century—; it is theological. The Jews, as a religious body that rejected the Messiah and participated in his death, bear a collective guilt that translates into juridical subordination. Baptism, which blots out guilt, would free them from servitude: that is why they are not forced to baptism, but that is also why they are not equated with Christians.

This doctrine had immediate canonical reception. The bull Sicut Judaeis of Calixtus II (1120), confirmed by Innocent III, Gregory IX and Innocent IV, articulated the double face: the Jews cannot be killed, nor forced to baptism, nor their synagogues profaned; but they live under papal protection, not on an equal footing. The Fourth Council of the Lateran (1215, canon 68) added the distinguishing sign —later the Castilian yellow «patch»— to prevent mixed sexual relations that, it was argued, deceived Christians. The Siete Partidas of Alfonso X (1256-1265, Partida VII, titles 24-25) codified subordination in Castile: no public office, no testifying against Christians, in separate aljamas.

The expulsion of 1492 was the logical culmination of this doctrine. If Spain was constituted by Catholic faith and the Jews were an unassimilated religious body, subordination ceased to be sustainable when massive crypto-Judaism demonstrated that assimilation was not working. The Crown did not exterminate them. It put them outside the Christian body, which was the political body. The doctrine of the witness protected their lives; the doctrine of subordination regulated their coexistence; when coexistence broke, the expulsion closed the cycle.

• Augustine, doctrine of the «witness» (Contra Faustum XII, 13; Enarrationes in Psalmos 59): the Jews must not be killed, because their dispersal testifies to Christian truth. Basis of all pre-conciliar doctrine.
• John Chrysostom, <em>Adversus Iudaeos</em> (c. 386-387): harshest patristic position. The synagogue as «den and lair of beasts». Tempered by Augustine, not abandoned.
• Calixtus II, bull <em>Sicut Judaeis</em> (1120): protected from violence, but in subordination. Confirmed by Innocent III, Gregory IX, Innocent IV.
• Thomas Aquinas, <em>Summa</em> II-II, q.10, a.8, <em>ad 2um</em>: the Jews, for deicide, are destined to perpetual servitude. Codified doctrine.
• Lateran IV (1215, canon 68): distinguishing sign to prevent mixtures. Siete Partidas (Alfonso X, Partida VII, titles 24-25): juridical subordination in Castile.
• Expulsion of 1492: logical culmination, not rupture. The unassimilated religious body is put outside when subordination ceases to be sustainable.

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«Synagoga Iudaeorum latrocinium est et bestiarum receptaculum.»

The synagogue of the Jews is a den of thieves and refuge of beasts.

John Chrysostom, Adversus Iudaeos I, 3 (c. 386-387). Patrologia Graeca 48, col. 847. Medieval Latin form.

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«Judaei autem, qui eum susceperunt, in servitutem perpetuam sunt deputati.»

The Jews, who received him [Christ], are destined to perpetual servitude.

Thomas Aquinas, Summa Theologica II-II, q.10, a.8, ad 2um. Leonine edition, Rome, 1899.

VIII.Legacy: the enduring distinction

The Thomist distinction between natural and judicial astrology has been extraordinarily influential. Its practical consequences include:

In contemporary astrology, many "non-determinist" practitioners unknowingly defend a position close to the Thomist one: the stars "incline, they do not compel". This formula, now commonplace, originates precisely from Saint Thomas Aquino.

• Allowed the development of scientific astronomy within the Church, because observing the heavens for natural purposes was licit and even meritorious (which is why the Church founded observatories like the Specola Vaticana, 1582).
• Prevented the acceptance of astrological determinism in Christian culture, unlike the medieval Islamic world, where judicial astrology enjoyed greater intellectual prestige.
• Defended human freedom and moral responsibility, a condition of possibility for all Catholic morality (sin, merit, salvation).
• Gave criteria to the Inquisition to pursue judicial astrology (especially political astrology, which sought to predict the deaths of kings and popes) without condemning astronomy or astrological medicine.

IX.The Counterpoint: Cecco d'Ascoli, the Burned Astrologer

The Thomist distinction was not merely a theoretical exercise: it had mortal consequences. Just a few decades after the death of Saint Thomas, his distinction was tested in the case of Cecco d'Ascoli (Francesco degli Stabili, c. 1257-1327), an Italian physician, astrologer, and Franciscan, professor in Bologna, author of the Commentarium in Sphaeram Joannis de Sacrobosco and the Acerba.

Cecco did not respect the Thomist boundary. In his works, he defended a radical astrological determinism: he went so far as to calculate the date of Christ's death from the stars, and asserted that celestial influence was practically irresistible even for the will. He crossed, without turning back, the line that Saint Thomas had drawn: from natural astrology to deterministic judicial astrology, and from there to theologically unacceptable theses (that the incarnation and passion were "written in the stars").

Condemned by the Inquisition in 1324, he abjured in Avignon before John XXII, but relapsed in his teachings in Bologna. Tried again, he was handed over to the secular arm and burned at the stake in Florence on September 26, 1327. His case became the canonical example—cited for centuries in manuals of moral theology—of the insurmountable limits of judicial astrology. The Inquisition, in condemning Cecco, was not condemning astronomy or astrological medicine (which continued to be taught in universities), but rather the deterministic pretension that denied freedom and divine sovereignty over history.

"Cecco d'Ascoli was the exemplary case: one who crosses the line that Saint Thomas drew between natural astrology and judicial astrology does not fall into an intellectual error, but into an error of faith."

X.Dante and Astrology: The Thomist Distinction in Literature

Contemporary of the canonization of Saint Thomas (1323), Dante Alighieri (1265-1321) is the main lay witness to how the Thomist distinction permeated the cultured society of his time. In the Divine Comedy, Dante unfolds a complete Ptolemaic cosmology —the concentric heavens governed by the planets, the empyrean beyond the sphere of the fixed stars— but subordinates all astral influence to free will, exactly as Saint Thomas had established.

The key passage is in Paradiso, canto II, where Beatrice explains to Dante that celestial influences are real, but that human will can resist them. The stars "incline" (inclinare) but do not "compel" (necessitare): the very Thomist formula. Dante expresses it in verse with a power that no scholastic treatise achieved:

«Lo cielo i vostri movimenti inizia; non dico tutti, ma, posto ch'i' 'l dica, lume v'è dato a bene e a malizia, e libero voler; che, se pure pctia ne i primi cimenti poi vince.»

The heaven initiates your movements; I do not say all, but, although I say it, light is given to you for good and for malice, and free will; that, if at first it suffers fatigue in the first trials, later it overcomes it.

Dante also personifies each heaven with a virtue: the Moon with faith, Mercury with hope, Venus with charity, the Sun with prudence, Mars with fortitude, Jupiter with justice, Saturn with temperance. This correspondence is not astrological in a judicial sense, but symbolic-moral: the stars are signs of an order willed by God, not determining causes of human destiny. It is natural and symbolic astrology, purged of all divinatory pretensions —exactly the operation that Saint Thomas had legitimized.

The literary triumph of the Divine Comedy showed European culture that the Ptolemaic-Aristotelian scheme, purified by Thomist theology, could be integrated into the Christian worldview without falling into determinism or superstition. That is why Dante, in canto X of Paradise, places Saint Thomas in the heaven of the Sun, among the great doctors, presenting him as the wise man who knew how to harmonize faith and reason, heaven and earth.

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««Lo cielo i vostri movimenti inizia; non dico tutti, ma, posto ch'i' 'l dica, lume v'è dato a bene e a malizia, e libero voler; che, se pure pctia ne i primi cimenti poi vince.»»

The heaven initiates your movements; I do not say all, but, although I say it, light is given to you for good and for malice, and free will; that, if at first it suffers fatigue in the first trials, later it overcomes it.

Paradiso II, vv. 13-18

XI.Chronology

XII.Sources and bibliography

• Saint Thomas Aquinas, Summa Theologica, II-II, question 95 ("On divination"), articles 1-8. BAC bilingual edition, Madrid.
• Saint Thomas Aquinas, Summa contra Gentiles, book III, caps. 70-87. BAC, Madrid.
• Saint Thomas Aquinas, Scriptum super Sententiis, book II, dist. 15, q.1, a.3.
• Saint Thomas Aquinas, De occultis operibus naturae (authenticity disputed, traditionally attributed).
• Roman Catechism (Catechismus Romanus, 1566), promulgated by Pius V after the Council of Trent. Part I, Commandments of the Decalogue.
• Bull Coeli et terrae of Sixtus V (1586), on the condemnation of judicial astrology.
• Dante Alighieri, Divine Comedy — Paradise, canto II (vv. 13-18, on free will and astral influence) and canto X (Saint Thomas in the heaven of the Sun).
• Cecco d'Ascoli, Acerba (condemned work, source of the inquisitorial process).
• Beltran, O. H. (2024). Thomas Aquinas and astrology. Dialogía, Journal of the Master's in History and Memory, UNLP (Argentina).
• Sertillanges, A.-D. (1910). Saint Thomas d'Aquin. Librairie Felix Alcan, Paris. (BAC Spanish edition: Sancto Thomas Aquinas, 2 vols.)
• Chenu, M.-D. (1950). Introduction à l'étude de saint Thomas d'Aquin. Institut d'Études Médiévales, Montreal/Paris.
• Thorndike, L. (1923-1958). A History of Magic and Experimental Science, vols. I-II. Columbia University Press. (For medieval astrological context.)
• Thorndike, L. (1929). The Sphere of Sacrobosco and Its Commentators. University of Chicago Press. (On Cecco d'Ascoli, pp. 142-176.)
• Corti, G. (1983). La felicità mentale. Nuove indagini sul signor Alighieri. Einaudi, Turin. (On Dante and medieval cosmology.)
• Denzinger-Hünermann. Compendium of the symbols, definitions and declarations of faith and morals. Bilingual edition, Herder.

XIII.Frequently Asked Questions

Saint Thomas did not condemn astrology outright, but rather distinguished between natural astrology (the study of astral influences on material bodies, licit) and judicial astrology (the pretense of predicting with certainty human free acts, condemned). This distinction, presented in the Summa Theologica II-II q.95, remains as the classical Catholic position on the subject.

Saint Thomas admitted that celestial bodies influence terrestrial bodies (heat, light, seasons, tides), and thus indirectly human passions and tendencies. However, he denied that the stars can determine human free will, which belongs to the spiritual order and escapes the material causality of the heavens.

According to classical Catholic doctrine, articulated by Saint Thomas, judicial astrology (that which claims to predict the future or free acts) constitutes a sin against the first commandment, as it usurps the divine prerogative of knowing the future and tends toward superstition. Natural astrology (astronomical observation for agricultural, medical, or meteorological purposes) is licit.

The main treatment is found in the Summa Theologica, II-II, question 95 ("On divination"), articles 2 to 5. It also addresses astral influence in Summa contra Gentiles III, caps. 70-87, and in the De occultis operibus naturae (a work of disputed authenticity but traditionally attributed).

Natural astrology, for Saint Thomas, is the knowledge of celestial movements applied to the prediction of physical phenomena: rains, droughts, harvests, tides, and in medicine, the critical periods of illnesses. It is licit because it is based on observable physical causes and does not claim to determine free will.

Because astronomy studies the movements of the stars as physical objects (licit and meritorious), while judicial astrology seeks to deduce human future from them (superstitious). The Thomistic distinction allowed the Church to promote astronomical science —it founded observatories such as the Vatican in 1582— without accepting astrological determinism.

Would you like to better understand your natal chart within this historical framework?