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St. Thomas Aquinas (c. 1225-1274), the eponym of Thomism. Picture by Fra Angelico (c. 1395-1455).

Thomism is the philosophical school that arose as a legacy of the work and thought of St. Thomas Aquinas. The word comes from the name of its originator, whose Summa Theologica was one of the most influential documents in medieval philosophy and continues to be studied today in philosophy classes. In the encyclical Doctoris Angelici, Pope St. Pius X cautioned that the teachings of the Church cannot be understood scientifically without the basic philosophical underpinnings of Aquinas' major thesis:

The capital theses in the philosophy of St. Thomas are not to be placed in the category of opinions capable of being debated one way or another, but are to be considered as the foundations upon which the whole science of natural and divine things is based; if such principles are once removed or in any way impaired, it must necessarily follow that students of the sacred sciences will ultimately fail to perceive so much as the meaning of the words in which the dogmas of divine revelation are proposed by the magistracy of the Church.[1]

The Second Vatican Council described Aquinas' system as the "Perennial Philosophy."[2]

Thomistic philosophy


Because St. Thomas Aquinas was a devout Catholic priest, he believed that truth is true wherever it is found, and thus consults Greek, Roman, Jewish, and Muslim philosophers. Specifically, he was a realist (i.e., that the world can be known as it is, in contrast to skepticism). He largely follows Aristotelian terminology and metaphysics, and wrote comprehensive commentaries on Aristotle, often affirming Aristotle's views with independent arguments. Aquinas also often quotes Aristotle as an authoritative source[3] (though never administering an argumentum ad verecundiam), referring to him as "the Philosopher."

Shortly before Thomas died, Reginald of Piperno implored him to finish his works. Thomas replied, "I cannot, because all that I have written seems like straw to me."[4] Apologist Peggy Frye of Catholic Answers comments that "Aquinas’s vision may have been a vision of heaven, compared to which everything else, no matter how glorious, seems worthless."[5]

Distinctive ideas

With the decree Postquam sanctissimus of 27 July 1914,[6] Pope St. Pius X declared that 24 theses formulated by "teachers from various institutions ... clearly contain the principles and more important thoughts" of Aquinas. They represent an admirable summary of Aquinas's system.


  1. Potency and Act divide being in such a way that whatever is, is either pure act, or of necessity it is composed of potency and act as primary and intrinsic principles.
  2. Since act is perfection, it is not limited except through a potency which itself is a capacity for perfection. Hence in any order in which an act is pure act, it will only exist, in that order, as a unique and unlimited act. But whenever it is finite and manifold, it has entered into a true composition with potency.
  3. Consequently, the one God, unique and simple, alone subsists in absolute being. All other things that participate in being have a nature whereby their being is restricted; they are constituted of essence and being, as really distinct principles.
  4. A thing is called a being because of "esse". God and creature are not called beings univocally, nor wholly equivocally, but analogically, by an analogy both of attribution and of proportionality.
  5. In every creature there is also a real composition of the subsisting subject and of added secondary forms, i.e. accidental forms. Such composition cannot be understood unless being is really received in an essence distinct from it.
  6. Besides the absolute accidents there is also the relative accident, relation. Although by reason of its own character relation does not signify anything inhering in another, it nevertheless often has a cause in things, and hence a real entity distinct from the subject.
  7. A spiritual creature is wholly simple in its essence. Yet there is still a twofold composition in the spiritual creature, namely, that of the essence with being, and that of the substance with accidents.
  8. However, the corporeal creature is composed of act and potency even in its very essence. These act and potency in the order of essence are designated by the names form and matter respectively.


  1. Neither the matter nor the form have being of themselves, nor are they produced or corrupted of themselves, nor are they included in any category otherwise than reductively, as substantial principles.
  2. Although extension in quantitative parts follows upon a corporeal nature, nevertheless it is not the same for a body to be a substance and for it to be quantified. For of itself substance is indivisible, not indeed as a point is indivisible, but as that which falls outside the order of dimensions is indivisible. But quantity, which gives the substance extension, really differs from the substance and is truly an accident.
  3. The principle of individuation, i.e., of numerical distinction of one individual from another with the same specific nature, is matter designated by quantity. Thus in pure spirits there cannot be more than individual in the same specific nature.
  4. By virtue of a body's quantity itself, the body is circumscriptively in a place, and in one place alone circumscriptively, no matter what power might be brought to bear.
  5. Bodies are divided into two groups; for some are living and others are devoid of life. In the case of the living things, in order that there be in the same subject an essentially moving part and an essentially moved part, the substantial form, which is designated by the name soul, requires an organic disposition, i.e. heterogeneous parts.


  1. Souls in the vegetative and sensitive orders cannot subsist of themselves, nor are they produced of themselves. Rather, they are no more than principles whereby the living thing exists and lives; and since they are wholly dependent upon matter, they are incidentally corrupted through the corruption of the composite.
  2. On the other hand, the human soul subsists of itself. When it can be infused into a sufficiently disposed subject, it is created by God. By its very nature, it is incorruptible and immortal.
  3. This rational soul is united to the body in such a manner that it is the only substantial form of the body. By virtue of his soul a man is a man, an animal, a living thing, a body, a substance and a being. Therefore, the soul gives man every essential degree of perfection; moreover, it gives the body a share in the act of being whereby it itself exists.
  4. From the human soul there naturally issue forth powers pertaining to two orders, the organic and the non-organic. The organic powers, among which are the senses, have the composite as their subject. The non-organic powers have the soul alone as their subject. Hence, the intellect is a power intrinsically independent of any bodily organ.
  5. Intellectuality necessarily follows upon immateriality, and furthermore, in such manner that the further the distance from matter, the higher the degree of intellectuality. Any being is the adequate object of understanding in general. But in the present state of union of soul and body, quantities abstracted from the material conditions of individuality are the proper object of the human intellect.
  6. Therefore, we receive knowledge from sensible things. But since sensible things are not actually intelligible, in addition to the intellect, which formally understands, an active power must be acknowledged in the soul, which power abstracts intelligible likeness or species from sense images in the imagination.
  7. Through these intelligible likenesses or species we directly know universals, i.e. the natures of things. We attain to singulars by our senses, and also by our intellect, when it beholds the sense images. But we ascend to knowledge of spiritual things by analogy.
  8. The will does not precede the intellect but follows upon it. The will necessarily desires that which is presented to it as a good in every respect satisfying the appetite. But it freely chooses among the many goods that are presented to it as desirable according to a changeable judgment or evaluation. Consequently, the choice follows the final practical judgment. But the will is the cause of it being the final one.


  1. We do not perceive by an immediate intuition that God exists, nor do we prove it a priori. But we do prove it a posteriori, i.e., from the things that have been created, following an argument from the effects to the cause: namely, from things which are moved and cannot be the adequate source of their motion, to a first unmoved mover; from the production of the things in this world by causes subordinated to one another, to a first uncaused cause; from corruptible things which equally might be or not be, to an absolutely necessary being; from things which more or less are, live, and understand, according to degrees of being, living and understanding, to that which is maximally understanding, maximally living and maximally a being; finally, from the order of all things, to a separated intellect which has ordered and organized things, and directs them to their end.
  2. The metaphysical motion of the Divine Essence is correctly expressed by saying that it is identified with the exercised actuality of its own being, or that it is subsistent being itself. And this is the reason for its infinite and unlimited perfection.
  3. By reason of the very purity of His being, God is distinguished from all finite beings. Hence it follows, in the first place, that the world could only have come from God by creation; secondly, that not even by way of a miracle can any finite nature be given creative power, which of itself directly attains the very being of any being; and finally, that no created agent can in any way influence the being of any effect unless it has itself been moved by the first Cause.


Aquinas says that the fundamental axioms of ontology are the principle of non-contradiction and the principle of causality. Therefore, any being that does not contradict these two laws could theoretically exist,[7] even if said being were incorporeal.[8]


Aquinas noted three forms of descriptive language when predicating: univocal, analogical, and equivocal.[9]

  • Univocality is the use of a descriptor in the same sense when applied to two objects or groups of objects. For instance, when the word "milk" is applied both to milk produced by cows and by any other female mammal.
  • Analogy, Aquinas maintained, occurs when a descriptor changes some but not all of its meaning. For example, the word "healthy" is analogical in that it applies both to a healthy person or animal (those that enjoy of good health) and to some food or drink (if it is good for the health).
  • Equivocation is the complete change in meaning of the descriptor and is an informal fallacy. For example, when the word "bank" is applied to river banks and financial banks. Modern philosophers talk of ambiguity.


[E]xistence is twofold: one is essential existence or the substantial existence of a thing, for example man exists, and this is existence simpliciter. The other is accidental existence, for example man is white, and this is existence secundum quid. (De Principiis Naturæ, 1.)

In Thomist philosophy, the definition of a being is "that which is," which is composed of two parts: "which" refers to its quiddity (literally "whatness"), and "is" refers to its esse (the Latin infinitive verb "to be").[10] "Quiddity" is synonymous with essence and nature; whereas "esse" refers to the principle of the being's existence. In other words, a being is "an essence that exists."[11]

The Catholic Encyclopedia summarizes Aquinas' definition of quiddity as "that which is expressed by its definition."[12] Further, the usage of "definition" that Aquinas gives is the genus of the being, plus a difference that sets it apart from the genus itself. For instance, the Aristotelian definition of "man" is "rational animal"; its genus being animal, and what sets apart man from other animals is his rationality.[13]

Being is divided in two ways: that which is in itself (substances), and that which is in another (accidents). Accidentals are qualities that apply to other things, such as shape or color: "[A]ccidents must include in their definition a subject which is outside their genus."[14] Because they only exist in other things, Aquinas holds that metaphysics is primarily the study of substances, as they are the primary mode of being.[15]

Essence is further categorized by substantial form: "[T]hrough the form, which is the actuality of matter, matter becomes something actual and something individual."[16] For instance, an animal's essence can be said to be whose matter is its body, and whose soul is its form.[17][18]

All real things have the transcendental properties of being: oneness, truth, goodness (that is, all things have a final cause and therefore a purpose), etc.[19]

Existence of God

Aquinas holds that the existence of God can be demonstrated by reason,[20] a view that is still taught by the Catholic Church.[21] The Quinque viae (Latin: five ways) found in the Summa Theologica (I, Q.2, art.4) are five arguments demonstrating the existence of God,[22] which today are categorized as:

1. Argumentum ex motu, or the argument of the unmoved mover;
2. Argumentum ex ratione causae efficientis, or the argument of the first cause;
3. Argumentum ex contingentia, or the argument from contingency;
4. Argumentum ex gradu, or the argument from degree; and
5. Argumentum ex fine, or the teleological argument.

Despite this, Aquinas also thought that sacred mysteries such as the Trinity could only be obtained through revelation; though these truths cannot contradict reason:

The existence of God and other like truths about God, which can be known by natural reason, are not articles of faith, but are preambles to the articles; for faith presupposes natural knowledge, even as grace presupposes nature, and perfection supposes something that can be perfected. Nevertheless, there is nothing to prevent a man, who cannot grasp a proof, accepting, as a matter of faith, something which in itself is capable of being scientifically known and demonstrated. (Summa, I, Q.2, art.2.)

Aquinas responds to the problem of evil by saying that God allows evil to exist that good may come of it,[23] (for goodness done out of free will is superior than goodness done from biological imperative) but does not personally cause evil Himself.[24] Gottfried Leibniz later expounds on this view[25] in his work Théodicée (1710), which argues that God has authored the best of all possible worlds, one that requires the existence of evil to be optimal.

View of God

Aquinas held the orthodox Christian view of God. Accordingly, he implicitly contradicted Aristotle's view of God as a being that is not omniscient; Aquinas held that not only did God have knowledge of everything,[26] God himself is knowledge.[27] Further, he held that because God is the first cause of the universe, it is consequently true that the universe is not eternal.[28]

God, according to Aquinas, is the sole being whose existence is the same as His essence: "what subsists in God is His existence."[29] (Hence why God names himself "I AM WHO AM" in Exodus 3:14.[30]) Consequently, God cannot be a body (that is, He cannot be composed of matter),[31] He cannot have any accidents,[32] and He must be simple (that is, not separated into parts; the Trinity is one substance in three persons).[33] Further, He is goodness itself,[34] perfect,[35] infinite,[36] omnipotent,[37] omniscient,[26] happiness itself,[38] knowledge itself,[27] love itself,[39] omnipresent,[40] immutable,[41] and eternal.[42] Summing up these properties, Aquinas offers the term actus purus (Latin: "to exist perfectly") as a composite.

Aquinas was an advocate of negative theology, which says that due to God's infinity, people can only speak of God in terms of analogy, for some of the aspects of the divine nature are hidden (Deus absconditus) and others revealed (Deus revelatus) to finite human minds. Thomist philosophy holds that we can know about God through his creation (general revelation), but only in an analogous manner.[43] For instance, we can speak of God's goodness only by understanding that goodness as applied to humans is similar to, but not identical with, the goodness of God. Further, he argues that sacred scripture employs figurative language: "Now it is natural to man to attain to intellectual truths through sensible objects, because all our knowledge originates from sense. Hence in Holy Writ, spiritual truths are fittingly taught under the likeness of material things."[44]

In order to demonstrate God's creative power, Aquinas says: "If a being participates, to a certain degree, in an 'accident,' this accidental property must have been communicated to it by a cause which possesses it essentially. Thus iron becomes incandescent by the action of fire. Now, God is His own power which subsists by itself. The being which subsists by itself is necessarily one."[45] This idea is also expounded by Bahya ibn Paquda in his Duties of the Heart.


Summa Theologiæ, Pars secunda, prima pars. (copy by Peter Schöffer, 1471)

In addition to agreeing with the Aristotelian definition of man as "the rational animal,"[13] Aquinas also held various other beliefs about the substance of man. For instance, as the essence (nature) of all men are the same,[46] and the definition of being is "an essence that exists,"[11] humans that are real therefore only differ by their specific qualities. More generally speaking, all beings of the same genus have the same essence, and so long as they exist, only differ by accidents and substantial form.[47]


Aquinas affirms Aristotle's definition of happiness as "an operation according to perfect virtue,"[48][49] and that "happiness is called man's supreme good, because it is the attainment or enjoyment of the supreme good."[50] Regarding what the virtues are, Aquinas ascertained the cardinal virtues to be prudence, temperance, justice, and fortitude. The cardinal virtues are natural and revealed in nature, and they are binding on everyone. There are, however, three theological virtues: faith, hope, and charity. These are supernatural and are distinct from other virtues in their object, namely, God.[51]

In accordance with Roman Catholic theology, Aquinas argues that humans can neither wish nor do good without divine grace.[52] Therefore, happiness is attained through the perseverance of virtue given by the Grace of God,[53] which is not fully attained on earth;[54] only at the beatific vision.[55][56] Notably, man cannot attain true happiness without God.[57]

Aquinas also held that there were intellectual virtues independent of morality or divine grace. Three of them are the speculative virtues of sophia (wisdom), episteme (science) and nous (understanding), which deal with what is; there are also techne (art), which is concerned with the man-made; and phronesis (prudence), which deals with practical actions.[58] Specifically, he says that "the intellectual virtues are habits by which the soul expresses the truth."[59]

Thomist ethics hold that it is necessary to observe both circumstances[60] and intention[61] to determine an action's moral value, and therefore Aquinas cannot be said to be strictly either a deontologicalist or a consequentialist. Rather, he would say that an action is morally good if it fulfills God's antecedent will.[62]

Of note is the principle of double effect, formulated in the Summa, II-II, Q.64, art.7, which is a justification of homicide in self-defense. Previously experiencing difficulties in the world of Christian philosophy, the doctrine of Just War was expounded by Aquinas with this principle. He says:

In order for a war to be just, three things are necessary. First, the authority of the sovereign by whose command the war is to be waged... Secondly, a just cause is required, namely that those who are attacked, should be attacked because they deserve it on account of some fault... Thirdly, it is necessary that the belligerents should have a rightful intention, so that they intend the advancement of good, or the avoidance of evil... (Summa, II-II, Q.40, art.1.)


Aquinas recognizes four different species of law, which he defines as "an ordinance of reason for the common good, made by him who has care of the community, and promulgated":[63]

1. Eternal law, which is "the type of Divine Wisdom, as directing all actions and movements;"[64]
2. Natural law, "whereby each one knows, and is conscious of, what is good and what is evil;"[65]
3. Human or temporal law, laws made by humans by necessity;[66] and
4. Divine law, which are moral imperatives specifically given through revelation.[67]

The development of natural law is one of the most influential parts of Thomist philosophy.[68] Aquinas says that "[the law of nature] is nothing other than the light of the intellect planted in us by God, by which we know what should be done and what should be avoided. God gave this light and this law in creation... For no one is ignorant that what he would not like to be done to himself he should not do to others, and similar norms."[69] This reflects St. Paul the Apostle's argument in Romans 2:15, that the "work of the law [is] written in [the Gentiles'] hearts, their conscience bearing witness to them."

Aquinas argues that the Mosaic covenant was divine, though rightfully only given to the Jews before Christ;[70] whereas the New Covenant replaces the Old Covenant[71] and is meant for all humans.[72]

Free will

Aquinas argues that there is no contradiction between God's providence and human free will:

... just as by moving natural causes [God] does not prevent their acts being natural, so by moving voluntary causes He does not deprive their actions of being voluntary: but rather is He the cause of this very thing in them; for He operates in each thing according to its own nature. (Summa, I., Q.83, art.1.)

Thomism is opposed to Molinism in how they view God's grace in relation to free will. Aquinas argued that God offers man cooperative grace, whereas Luis de Molina held that God's grace was given based on a middle knowledge.


"Whatever is in our intellect must have previously been in the senses."
St. Thomas Aquinas, the peripatetic axiom.[73]

Aquinas adhered to the correspondence theory of truth, which says that something is true "when it conforms to the external reality."[74] Therefore, any being that exists can be said to be true insofar that it participates in the world.[75]

Aristotle's De anima (On the Soul) divides the mind into three parts: sensation, imagination and intellection. When one perceives an object, his mind composites a sense-image. When he remembers the object he previously sensed, he is imagining its form (the image of the imagination is often translated as "phantasm"). When he extracts information from this phantasm, he is using his intellect.[76] Consequently, all human knowledge concerning universals (such as species and attributes) are derived from the phantasm ("the received is in the receiver according to the mode of the receiver"[77]), which itself is a recollection of an experience. Concerning the question of "Whether the intellect can actually understand through the intelligible species of which it is possessed, without turning to the phantasms?" in the Summa Theologica, Aquinas quotes Aristotle in the sed contra: "the soul understands nothing without a phantasm."[78] Hence the peripatetic axiom. (Another conclusion to be drawn from this is that error is a result of drawing false conclusions based on our sensations.)[79]

Aquinas' epistemological theory would later be classified as empiricism, for holding that sensations are a necessary step in acquiring knowledge, and that deductions cannot be made from pure reason.[80]

Impact of Thomism

Saint Thomas was important in shifting the influence of scholastic medieval philosophy away from neoplatonism and towards Aristotle. In this he was influenced by contemporary Islamic philosophy, especially the work of Averroes. The ensuing school of thought, through its influence on Catholicism and the ethics of the Catholic school, is by any standard one of the most influential philosophies of all time, also significant due to the sheer number of people living by its teachings.

Thomism's affirmation was not at all easy and quick. Even before Aquinas' death, Stephen Tempier, Bishop of Paris, forbade certain positions associated with Aquinas (especially Aquinas' denial of both universal hylomorphism and a plurality of substantial forms in a single substance) to be taught in the Faculty of Arts at Paris. Through the influence the more traditional Augustinian theologians, some theses of Aquinas were condemned in 1277 by the ecclesiastical authorities of Paris and Oxford (the most important theological schools in Middle Age Europe). The Franciscan Order vehemently opposed the ideas of the Dominican Thomas, while the Dominicans quickly and institutionally took up the defense of his work (1286), and soon thereafter adopted it as an official philosophy of the order to be taught in their studia. Early opponents of Aquinas include William de la Mare, Henry of Ghent, Giles of Rome, and Jon Duns Scotus.

Early, noteworthy defenders of Aquinas were his former teacher St. Albertus Magnus, the ill-fated Richard Knapwell, William Macclesfeld, Giles of Lessines, John of Quidort (a lay master), Bernard of Auvergne, and Thomas of Sutton. The canonization of St. Thomas Aquinas in 1323 led to revoking the condemnation of 1277. Later, Aquinas and his school would find a formidable opponent in the via moderna, particularly in William of Ockham and his adherents.

Thomism remained for quite a long time a doctrine held principally by Dominican theologians, such as Giovanni Capreolo (1380–1444) or Tommaso de Vio (1468–1534). Eventually, in the 16th century, Thomas found a stronghold on the Iberian Peninsula, through for example the Dominicans Francisco de Vitoria (particularly noteworthy for his work in natural law theory), Domingo de Soto (notable for his work on economic theory), John of St. Thomas, and Domingo Báñez; the Carmelites of Salamanca (i.e., the Salmanticenses); and even, in a way, the newly formed Jesuits, particularly Francisco Suárez, and Luis de Molina.

The Modern Period brought considerable difficulty for Thomism.[81] By the 19th century, Aquinas' theological doctrine was often presented in seminaries through his Jesuit manualist interpreters, who often adopted his theology in an eclectic way, while his philosophy was often neglected altogether in favor of modern philosophers. And in all this, the Dominican Order, was having demographic difficulties. Pope Leo XIII attempted a Thomistic revival, particularly with his 1879 encyclical Aeterni Patris and his establishment of the Leonine Commission, established to produce critical editions of Aquinas's opera omnia. This encyclical served as the impetus for the rise of Neothomism, which brought an emphasis on the ethical parts of Thomism, as well as a large part of its views on life, humans, and theology, are found in the various schools of Neothomism (which arose in response to the 1879 encyclical Aeterni Patris encouraging the revival of Thomism). Neothomism held sway as the dominant philosophy of the Roman Catholic Church until the Second Vatican Council, which seemed to confirm the significance of Ressourcement theology. Thomism remains a vibrant and challenging school of philosophy today, and influential in Catholicism, though "The Church has no philosophy of her own nor does she canonize any one particular philosophy in preference to others."[82] According to one of its most famous and controversial proponents, Alasdair MacIntyre, a Thomistic Aristotelianism is the best philosophical theory so far of our knowledge of external reality and of our own practice.

Influence on Jewish thought

Aquinas's doctrines, because of their close relationship with those of Jewish philosophy, found great favor among Jews. Judah Romano (born 1286) translated Aquinas's ideas from Latin into Hebrew under the title Ma'amar ha-Mamschalim, together with other small treatises extracted from the "Contra Gentiles" ("Neged ha-Umot").

Eli Hobillo (1470) translated, without the Hebrew title, the "Quæstiones Disputatæ," "Quæstio de Anima," his "De Animæ Facultatibus," under the title "Ma'amar be-KoḦot ha-Nefesh," (edited by Jellinek); his "De Universalibus" as "Be-Inyan ha-Kolel"; "Shaalot Ma'amar beNimẓa we-biMehut."

Abraham Nehemiah ben Joseph (1490) translated Saint Thomas' "Commentarii in Metaphysicam." According to Moses Almosnino, Isaac Abravanel desired to translate the "Quæstio de Spiritualibus Creaturis." Abravanel indeed seems to have been well acquainted with the philosophy of Saint Thomas Aquinas, whom he mentions in his work "Mif'alot Elohim" (vi. 3). The physician Jacob Zahalon (d. 1693) translated some extracts from the Summa contra Gentiles.

Connection with Jewish thought

Aquinas did not disdain to draw upon Jewish philosophical sources. His main work, the Summa Theologica, shows a profound knowledge not only of the writings of Avicebron (Ibn Gabirol), whose name he mentions, but also of most Jewish philosophical works then existing.

Aquinas pronounces himself energetically against the hypothesis of the eternity of the world, in agreement with both Christian and Jewish theology. But as this theory is attributed to Aristotle, he seeks to demonstrate that the latter did not express himself categorically on this subject. "The argument," said he, "which Aristotle presents to support this thesis is not properly called a demonstration, but is only a reply to the theories of those ancients who supposed that this world had a beginning and who gave only impossible proofs. There are three reasons for believing that Aristotle himself attached only a relative value to this reasoning..."[83] In this Aquinas paraphrases Maimonides' Guide for the Perplexed, where those reasons are given.[84]

Scholarly perspectives on Thomism

René Descartes

Thomism began to decline in popularity in the modern period,[81] which was inaugurated by René Descartes' works Discourse on the Method in 1637 and Meditations on First Philosophy in 1641. The Cartesian doctrines of mind-body dualism and the fallibility of the senses implicitly contradicted Aristotle and Aquinas:

But, meanwhile, I feel greatly astonished when I observe [the weakness of my mind, and] its proneness to error. For although, without at all giving expression to what I think, I consider all this in my own mind, words yet occasionally impede my progress, and I am almost led into error by the terms of ordinary language. We say, for example, that we see the same wax when it is before us, and not that we judge it to be the same from its retaining the same color and figure: whence I should forthwith be disposed to conclude that the wax is known by the act of sight, and not by the intuition of the mind alone, were it not for the analogous instance of human beings passing on in the street below, as observed from a window. In this case I do not fail to say that I see the men themselves, just as I say that I see the wax; and yet what do I see from the window beyond hats and cloaks that might cover artificial machines, whose motions might be determined by springs ? But I judge that there are human beings from these appearances, and thus I comprehend, by the faculty of judgment alone which is in the mind, what I believed I saw with my eyes. (Meditations on First Philosophy, Med. II, §13.)

G. K. Chesterton

In describing Thomism as a philosophy of common sense, G. K. Chesterton wrote:

Since the modern world began in the sixteenth century, nobody's system of philosophy has really corresponded to everybody's sense of reality; to what, if left to themselves, common men would call common sense. Each started with a paradox; a peculiar point of view demanding the sacrifice of what they would call a sane point of view. That is the one thing common to Hobbes and Hegel, to Kant and Bergson, to Berkeley and William James. A man had to believe something that no normal man would believe, if it were suddenly propounded to his simplicity; as that law is above right, or right is outside reason, or things are only as we think them, or everything is relative to a reality that is not there. The modern philosopher claims, like a sort of confident man, that if we will grant him this, the rest will be easy; he will straighten out the world, if he is allowed to give this one twist to the mind...
Against all this the philosophy of St. Thomas stands founded on the universal common conviction that eggs are eggs. The Hegelian may say that an egg is really a hen, because it is a part of an endless process of Becoming; the Berkelian may hold that poached eggs only exist as a dream exists, since it is quite as easy to call the dream the cause of the eggs as the eggs the cause of the dream; the Pragmatist may believe that we get the best out of scrambled eggs by forgetting that they ever were eggs, and only remembering the scramble. But no pupil of St. Thomas needs to addle his brains in order adequately to addle his eggs; to put his head at any peculiar angle in looking at eggs, or squinting at eggs, or winking the other eye in order to see a new simplification of eggs. The Thomist stands in the broad daylight of the brotherhood of men, in their common consciousness that eggs are not hens or dreams or mere practical assumptions; but things attested by the Authority of the Senses, which is from God. (Chesterton, St. Thomas Aquinas, p. 136).

See also


  1. Pope St. Pius X, Doctoris Angelici (29th June 1914).
  2. Second Vatican Council, Optatam Totius (28th October 1965) 15.
  3. E.g., Summa, Q.84, art.7., where the sed contra is only a quote from Aristotle's De anima.
  4. Davies, Brian (1993). The Thought of Thomas Aquinas, p. 9. Oxford University Press.
  5. Peggy Frye, This Rock Volume 17 (May-June 2006), Number 5, Quick Questions.
  6. Postquam sanctissimus, Latin with English translation
  7. De Ente et Essentia, 67-68. "Although everyone admits the simplicity of the First Cause, some try to introduce a composition of matter and form in the intelligences and in souls... But this is not in agreement with what philosophers commonly say, because they call them substances separated from matter, and prove them to be without all matter."
  8. Summa contra Gentiles, II, chp. 91.
  9. Sproul, R.C. (1998). Renewing Your Mind: Basic Christian Beliefs You Need to Know. Grand Rapids, MI: Baker Books. p. 33. ISBN 0-8010-5815-5. 
  10. De Ente et Essentia, 83. "And this is why substances of this sort are said by some to be composed of “that by which it is” and “that which is,” or as Boethius says, of “that which is” and “existence.”"
  11. 11.0 11.1 Summa, I, Q.3, art.4. "Therefore, if the existence of a thing differs from its essence, this existence must be caused either by some exterior agent or by its essential principles."
  12. Aveling, Francis. "Essence and Existence." The Catholic Encyclopedia. Vol. 5. New York: Robert Appleton Company, 1909. 4 Nov. 2009.
  13. 13.0 13.1 De Ente et Essentia, 37.
  14. De Ente et Essentia, 17.
  15. Ibid., 110. "And because accidents are not composed of matter and form, their genus cannot be taken from matter and their difference from form, as in the case of composed substances."
  16. De Ente et Essentia, 18.
  17. De Ente et Essentia, 14.
  18. De Principiis Naturæ, 5. "But, just as everything which is in potency can be called matter, so also everything from which something has existence whether that existence be substantial or accidental, can be called form; for example man, since he is white in potency, becomes actually white through whiteness, and sperm, since it is man in potency, becomes actually man through the soul."
  19. De veritate, Q.1.
  20. Summa, I, Q.2.
  21. Catechism of the Catholic Church, CCC 34.
  22. Aquinas offers more metaphysical explanations for the existence of God in De Ente et Essentia and elsewhere, though the Quinquae viae are the most well-known and most commonly analyzed among these.
  23. Summa Theologiae, I, q. 2, a. 3 ad 2
  24. Summa, I, Q.49, art.2.
  25. Gottfried W. Leibniz, Discourse on Metaphysics, XI. "The many investigations which I carried on compelled me to recognize that our moderns do not do sufficient justice to Saint Thomas and to the other great men of that period and that there is in the theories of the scholastic philosophers and theologians far more solidity than is imagined, provided that these theories are employed a propos and in their place."
  26. 26.0 26.1 Summa, I, Q.14, arts. 5, 6, & 9.
  27. 27.0 27.1 Ibid., art. 1.
  28. Summa, I, Q.10, art. 3. "Jerome says that 'God is the only one who has no beginning.' Now whatever has a beginning, is not eternal. Therefore God is the only one eternal."
  29. Summa, I, Q.3, art.4.
  30. Summa, I, Q.13, art.11.
  31. Ibid., art. 1.
  32. Ibid., art. 6.
  33. Ibid., art. 7.
  34. Summa, I., Q.6., art.2 & 3.
  35. Summa, I., Q.4.
  36. Summa, I., Q.7.
  37. Summa, I., Q.25, art.3.
  38. Summa, II-I, Q.3, art.1. "God is happiness by His Essence."
  39. Summa, I, Q.20, art.1.
  40. Summa, I., Q.8.
  41. Summa, I., Q.9.
  42. Summa, I., Q.10, art.2.
  43. Summa contra Gentiles, Bk. I, chp. 30. "For we cannot grasp what God is, but only what He is not and how other things are related to Him, as is clear from what we said above."
  44. Summa, I, Q.1, art.9.
  45. Summa, I, Q.44, art.1.
  46. De Ente et Essentia, 24. "It is clear, therefore, that the essence of man and the essence of Socrates do not differ, except as the non-designated from the designated. Whence the Commentator says in his considerations on the seventh book of the Metaphysics that “Socrates is nothing other than animality and rationality, which are his quiddity.”"
  47. De Ente et Essentia, 33. "The difference, on the contrary, is a name taken from a determinate form, and taken in a determinate way, i.e. as not including a determinate matter in its meaning. This is clear, for example, when we say animated, i.e., that which has a soul; for what it is, whether a body or something other, is not expressed. Whence Ibn Sīnā says that the genus is not understood in the difference as a part of its essence, but only as something outside its essence, as the subject also is understood in its properties. And this is why the genus is not predicated essentially of the difference, as the Philosopher says in the third book of the Metaphysics and in the fourth book of the Topics, but only in the way in which a subject is predicated of its property."
  48. Summa, II-I, Q.3, art.2.
  49. St. Thomas Aquinas' commentary on Nicomachean Ethics, Lec. 10, §130. Aquinas further says that "it is clear that happiness is a virtue-oriented activity proper to man in a complete life."
  50. Summa, II-I, Q.3, art.1.
  51. Summa, II-I, Q.62, art.2.
  52. Summa, II-I, Q.109, art.2.
  53. Summa, II-I, Q. 109, art.10.
  54. Summa, II-I, Q.5, art.3. "First, from the general notion of happiness. For since happiness is a "perfect and sufficient good," it excludes every evil, and fulfils every desire. But in this life every evil cannot be excluded."
  55. Summa, II-I, Q.5, art.1. "Happiness is the attainment of the Perfect Good... And therefore man can attain Happiness. This can be proved again from the fact that man is capable of seeing God, [which] man's perfect Happiness consists."
  56. Summa, supp., Q.93, art.1.
  57. Summa, II-I, Q.5, art.5.
  58. Summa, II-I, Q. 57.
  59. St. Thomas Aquinas' commentary on the Nicomachean Ethics, Bk. VI, Lec. 3, §1143.
  60. Summa, II-I, Q.18, art.3 & 10
  61. Summa, II-I, Q.19, art.1 & 2.
  62. De veritate, Q. 23, art.7.
  63. Summa, II-I, Q.90, art.4.
  64. Summa, II-I, Q.93, art. 1.
  65. St. Thomas Aquinas cites Romans 2:14 authoritatively on the definition of natural law, in Summa, II-I, Q.91, art.2.
  66. Summa, II-I, Q.95, art.1.
  67. Summa, II-I, Q.91, art.4. "By the natural law the eternal law is participated proportionately to the capacity of human nature. But to his supernatural end man needs to be directed in a yet higher way. Hence the additional law given by God, whereby man shares more perfectly in the eternal law."
  68. Cf. Veritatis splendor, 12.
  69. St. Thomas Aquinas' commentary on the Ten Commandments, prologue, sec. 'A fourfold law'.
  70. Summa, II-I, Q.98, art.1 & 4-5.
  71. Summa, II-I, Q.107, art.2.
  72. Summa, II-I, Q.106, art.4.
  73. De veritate, Q.2, art.3, answer 19.
  74. De veritate, Q. 1, art. 3.
  75. Summa, I, Q.16, art.6.
  76. De anima, Bk. II, Chp. V, 417b18–418a25.
  77. Summa, I, Q.84, art.1.
  78. Summa, I, Q.84, art.7.
  79. St. Thomas Aquinas' commentary on De anima, §688.
  80. Summa, I, Q.84, art.8.
  81. 81.0 81.1 Kennedy, Daniel. "Thomism." The Catholic Encyclopedia. Vol. 14. New York: Robert Appleton Company, 1912. 8 Nov. 2009. "Gradually, however, during the seventeenth and eighteenth centuries, there came a decline in the study of the works of the great Scholastics."
  82. Fides et ratio, 49.
  83. Summa, I, Q.46., art.1.
  84. Maimonides, The Guide for the Perplexed, (I:2,15).

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