The following is from the New Schaff–Herzog Encyclopedia of Religious Knowledge (a public-domain work):
The Summa, Part I: Theology
St. Thomas's greatest work was the Summa, and it is the fullest presentation of his views. He worked on it from the time of Clement IV (after 1265) until the end of his life. When he died, he had reached Question 90 of Part III (on the subject of penance). What was lacking was added afterwards from the fourth book of his commentary on the Sentences of Peter Lombard as a supplementum, which is not found in manuscripts of the thirteenth and fourteenth centuries. The Summa was translated into Greek (apparently by Maximus Planudes around 1327), Armenian, many European languages, and Chinese.
It consists of three parts. Part I treats of God, who is the "first cause, himself uncaused" (primum movens immobile) and as such existent only in act (actu) – that is, pure actuality without potentiality, and therefore without corporeality. His essence is actus purus et perfectus. This follows from the fivefold proof for the existence of God; namely, there must be a first mover, unmoved, a first cause in the chain of causes, an absolutely necessary being, an absolutely perfect being, and a rational designer. In this connection the thoughts of the unity, infinity, unchangeability, and goodness of the highest being are deduced.
As God rules in the world, the "plan of the order of things" preexists in him; in other words, his providence and the exercise of it in his government are what condition as cause everything which comes to pass in the world. Hence follows predestination: from eternity some are destined to eternal life, while as concerns others "he permits some to fall short of that end". Reprobation, however, is more than mere foreknowledge; it is the "will of permitting anyone to fall into sin and incur the penalty of condemnation for sin". The effect of predestination is grace. Since God is the first cause of everything, he is the cause of even the free acts of men through predestination. Determinism is deeply grounded in the system of St. Thomas; things (with their source of becoming in God) are ordered from eternity as means for the realization of his end in himself. On moral grounds, St. Thomas advocates freedom energetically; but, with his premises, he can have in mind only the psychological form of self-motivation. Nothing in the world is accidental or free, although it may appear so in reference to the proximate cause. From this point of view, miracles become necessary in themselves and are to be considered merely as inexplicable to man. From the point of view of the first cause, all is unchangeable, although from the limited point of view of the secondary cause, miracles may be spoken of.
In his doctrine of the Trinity, Aquinas starts from the Augustinian system. Since God has only the functions of thinking and willing, only two processiones can be asserted from the Father; but these establish definite relations of the persons of the Trinity, one to another. The relations must be conceived as real and not as merely ideal; for, as with creatures relations arise through certain accidents, since in God there is no accident but all is substance, it follows that "the relation really existing in God is the same as the essence according to the thing". From another side, however, the relations as real must be really distinguished one from another. Therefore, three persons are to be affirmed in God.
Man stands opposite to God; he consists of soul and body. The "intellectual soul" consists of intellect and will. Furthermore, the soul is the absolutely indivisible form of man; it is immaterial substance, but not one and the same in all men (as the Averroists assumed). The soul's power of knowing has two sides: a passive (the intellectus possibilis) and an active (the intellectus agens). It is the capacity to form concepts and to abstract the mind's images (species) from the objects perceived by sense; but since what the intellect abstracts from individual things is universal, the mind knows the universal primarily and directly and knows the singular only indirectly by virtue of a certain reflexio (compare Scholasticism). As certain principles are immanent in the mind for its speculative activity, so also a "special disposition of works" – or the synderesis (rudiment of conscience) – is inborn in the "practical reason", affording the idea of the moral law of nature so important in medieval ethics.
The Summa, Part II: Ethics
Part II of the Summa is divided into two parts. The first part comprises 114 quaestiones, and the second part comprises 189. The two parts of the second part are usually presented as containing several "treatises". The contents are as follows:
First part of Part II:
- Treatise on the last end (qq. 1 to 5)
- Treatise on human acts: Acts peculiar to humans (qq. 6 to 21)
- Treatise on the passions (qq. 22 to 48)
- Treatise on habits (qq. 49 to 54)
- Treatise on habits in particular (qq. 55 to 89): Good habits, i.e., virtues (qq. 55 to 70)
- Treatise on law (qq. 90 to 108)
- Treatise on grace (qq. 109 to 114)
Second part of Part II:
- Treatise on the theological virtues (qq. 1 to 46)
- Treatise on the cardinal virtues (qq. 47 to 170)
- Treatise on prudence (qq. 47 to 56)
- Treatise on justice (qq. 57 to 122)
- Treatise on fortitude and temperance (qq. 123 to 170)
- Treatise on gratuitous graces (qq. 171 to 182)
- Treatise on the states of life (qq. 183 to 189)
Content in general
The first part of the Summa is summed up in the premise that God governs the world as the "universal first cause". God sways the intellect; he gives the power to know and impresses the species intelligibiles on the mind, and he sways the will in that he holds the good before it as aim, creating the virtus volendi. "To will is nothing else than a certain inclination toward the object of the volition which is the universal good." God works all in all, but so that things also themselves exert their proper efficiency. Here the Areopagitic ideas of the graduated effects of created things play their part in St. Thomas's thought. The second part of the Summa (two parts, Prima Secundae and Secunda Secundae) follows this complex of ideas. Its theme is man's striving for the highest end, which is the blessedness of the visio beata. Here, St. Thomas develops his system of ethics, which has its root in Aristotle.
In a chain of acts of will, man strives for the highest end. They are free acts, insofar as man has in himself the knowledge of their end (and therein the principle of action). In that the will wills the end, it wills also the appropriate means, chooses freely and completes the consensus. Whether the act is good or evil depends on the end. The "human reason" pronounces judgment concerning the character of the end; it is, therefore, the law for action. Human acts, however, are meritorious insofar as they promote the purpose of God and his honor.
By repeating a good action, man acquires a moral habit or a quality that enables him to do the good gladly and easily. This is true, however, only of the intellectual and moral virtues (which St. Thomas treats after the manner of Aristotle); the theological virtues are imparted by God to man as a "disposition", from which the acts here proceed; while they strengthen, they do not form it. The "disposition" of evil is the opposite alternative. An act becomes evil through deviation from the reason and from divine moral law. Therefore, sin involves two factors: its substance (or matter) is lust; in form, however, it is deviation from the divine law.
Sin has its origin in the will, which decides (against reason) for a "changeable good". Since, however, the will also moves the other powers of man, sin has its seat in these too. By choosing such a lower good as its end, the will is misled by self-love, so that this works as cause in every sin. God is not the cause of sin since, on the contrary, he draws all things to himself; but from another side, God is the cause of all things, so he is efficacious also in sin as actio but not as ens. The devil is not directly the cause of sin, but he incites the imagination and the sensuous impulse of man (as men or things may also do).
Sin is original sin. Adam's first sin passes through himself to all the succeeding race; because he is the head of the human race and "by virtue of procreation human nature is transmitted and along with nature its infection." The powers of generation are, therefore, designated especially as "infected". The thought is involved here by the fact that St. Thomas, like other scholastics, believed in creationism; he therefore taught that souls are created by God. Two things, according to St. Thomas, constituted man's righteousness in paradise – the justitia originalis, or the harmony of all man's powers before they were blighted by desire, and the possession of the gratis gratum faciens (the continuous, indwelling power of good). Both are lost through original sin, which, in form, is the "loss of original righteousness." The consequence of this loss is the disorder and maiming of man's nature, which shows itself in "ignorance; malice, moral weakness, and especially in concupiscentia, which is the material principle of original sin." The course of thought here is as follows: when the first man transgressed the order of his nature appointed by nature and grace, he (and with him the human race) lost this order. This negative state is the essence of original sin. From it follow an impairment and perversion of human nature in which thenceforth lower aims rule, contrary to nature, and release the lower element in man.
Since sin is contrary to the divine order, it is guilt and subject to punishment. Guilt and punishment correspond to each other; and since the "apostasy from the invariable good which is infinite," fulfilled by man, is unending, it merits everlasting punishment.
God works even in sinners to draw them to the end by "instructing through the law and aiding by grace." The law is the "precept of the practical reason." As the moral law of nature, it is the participation of the reason in the all-determining "eternal reason"; but since man falls short in his appropriation of this law of reason, there is need of a "divine law"; and since the law applies to many complicated relations, the practicae dispositiones of the human law must be laid down.
The divine law consists of an old and a new. Insofar as the old divine law contains the moral law of nature, it is universally valid; what there is in it, however, beyond this is valid only for the Jews. The new law is "primarily grace itself" and so a "law given within"; "a gift superadded to nature by grace," but not a "written law." In this sense, as sacramental grace, the new law justifies. It contains, however, an "ordering" of external and internal conduct and so regarded is, as a matter of course, identical with both the old law and the law of nature. The consilia show how one may attain the end "better and more expediently" by full renunciation of worldly goods.
Since man is sinner and creature, he needs grace to reach the final end. The "first cause" alone is able to reclaim him to the "final end." This is true after the fall, although it was needful before. Grace is, on one side, "the free act of God", and, on the other side, the effect of this act, the gratia infusa or gratia creata, a habitus infusus that is instilled into the "essence of the soul," "a certain gift of disposition, something supernatural proceeding from God into man." Grace is a supernatural ethical character created in man by God, which comprises in itself all good, both faith and love.
Justification by grace comprises four elements: "the infusion of grace, the influencing of free will toward God through faith, the influencing of free will respecting sin, and the remission of sins." It is a "transmutation of the human soul," and takes place "instantaneously". A creative act of God enters, which, however, executes itself as a spiritual motive in a psychological form corresponding to the nature of man. Semipelagian tendencies are far removed from St. Thomas. In that man is created anew, he believes and loves, and now, sin is forgiven. Then begins good conduct; grace is the "beginning of meritorious works." Aquinas conceives of merit in the Augustinian sense: God gives the reward for that toward which he himself gives the power. Man can never of himself deserve the prima gratis," nor meritum de congruo (by natural ability; cf. R. Seeberg, Lehrbuch der Dogmengeschichte, ii. 105–106, Leipsic, 1898).
After thus stating the principles of morality, in the Secunda Secundae, St. Thomas comes to a minute exposition of his ethics according to the scheme of the virtues. The conceptions of faith and love are of much significance in the complete system of St. Thomas. Man strives toward the highest good with the will or through love; but since the end must first be "apprehended in the intellect," knowledge of the end to be loved must precede love; "because the will can not strive after God in perfect love unless the intellect have true faith toward him." Inasmuch as this truth that is to be known is practical, it first incites the will, which then brings the reason to "assent"; but since, furthermore, the good in question is transcendent and inaccessible to man by himself, it requires the infusion of a supernatural "capacity" or "disposition" to make man capable of faith as well as love. Accordingly, the object of both faith and love is God, involving also the entire complex of truths and commandments that God reveals, insofar as they in fact relate to God and lead to him. Thus, faith becomes recognition of the teachings and precepts of the Scriptures and the Church ("the first subjection of man to God is by faith"). The object of faith, however, is, by its nature, object of love; therefore, faith comes to completion only in love ("by love is the act of faith accomplished and formed").
Treatise on Law
According to Question 90, Article Four of the Second Part of the Summa, law "is nothing else than an ordinance of reason for the common good, made by him who has care of the community, and promulgated." All law comes from the eternal law of Divine Reason that governs the universe, which is understood and participated in by rational beings (such as men and angels) as the natural law. The natural law, when codified and promulgated, is the human law. In addition to the human law, dictated by reason, man also has the divine law, which, according to Question 91, is dictated through revelation, that man may be "directed how to perform his proper acts in view of his last end," "that man may know without any doubt what he ought to do and what he ought to avoid," because "human law could not sufficiently curb and direct interior acts," and since "human law cannot punish or forbid all evil deeds: since while aiming at doing away with all evils, it would do away with many good things, and would hinder the advance of the common good, which is necessary for human intercourse." Human law is not all-powerful; it cannot govern a man's conscience, nor prohibit all vices, nor can it force all men to act according to its letter, rather than its spirit. Furthermore, it is possible that an edict can be issued without any basis in law as defined in Question 90; in this case, men are under no compulsion to act, save as it helps the common good. This separation between law and acts of force also allows men to depose tyrants, or those who flout the natural law; while removing an agent of the law is contrary to the common good and the eternal law of God, which orders the powers that be, removing a tyrant is lawful as he has ceded his claim to being a lawful authority by acting contrary to law.
The Summa, Part III: Christ
The way which leads to God is Christ, the theme of Part III. It can be asserted that the incarnation was absolutely necessary. The Unio between the Logos and the human nature is a "relation" between the divine and the human nature, which comes about by both natures being brought together in the one person of the Logos. An incarnation can be spoken of only in the sense that the human nature began to be in the eternal hypostasis of the divine nature. So Christ is unum since his human nature lacks the hypostasis. The person of the Logos, accordingly, has assumed the impersonal human nature, and in such way that the assumption of the soul became the means for the assumption of the body. This union with the human soul is the gratia unionis, which leads to the impartation of the gratia habitualis from the Logos to the human nature. Thereby, all human potentialities are made perfect in Jesus. Besides the perfections given by the vision of God, which Jesus enjoyed from the beginning, he receives all others by the gratia habitualis. Insofar, however, as it is the limited human nature which receives these perfections, they are finite. This holds both of the knowledge and the will of Christ. The Logos impresses the species intelligibiles of all created things on the soul, but the intellectus agens transforms them gradually into the impressions of sense. On another side, the soul of Christ works miracles only as instrument of the Logos, since omnipotence in no way appertains to this human soul in itself. Concerning redemption, St. Thomas teaches that Christ is to be regarded as redeemer after his human nature but in such way that the human nature produces divine effects as organ of divinity. The one side of the work of redemption consists herein, that Christ as head of humanity imparts ordo, perfectio, and virtus to his members. He is the teacher and example of humanity; his whole life and suffering as well as his work after he is exalted serve this end. The love wrought hereby in men effects, according to Luke vii. 47, the forgiveness of sins.
This is the first course of thought. Then follows a second complex of thoughts, which has the idea of satisfaction as its center. To be sure, God as the highest being could forgive sins without satisfaction; but because his justice and mercy could be best revealed through satisfaction, he chose this way. As little, however, as satisfaction is necessary in itself, so little does it offer an equivalent, in a correct sense, for guilt; it is rather a "superabundant satisfaction", since on account of the divine subject in Christ in a certain sense his suffering and activity are infinite. With this thought, the strict logical deduction of Anselm's theory is given up. Christ's suffering bore personal character in that it proceeded "out of love and obedience". It was an offering brought to God, which as a personal act had the character of merit. Thereby, Christ "merited" salvation for men. As Christ, exalted, still influences men, so does he still work on their behalf continually in heaven through the intercession (interpellatio). In this way, Christ as head of humanity effects the forgiveness of their sins, their reconciliation with God, their immunity from punishment, deliverance from the devil, and the opening of heaven's gate; but inasmuch as all these benefits are already offered through the inner operation of the love of Christ, Aquinas has combined the theories of Anselm and Abelard by joining the one to the other.
The doctrine of the sacraments follows the Christology; the sacraments "have efficacy from the incarnate Word himself". They are not only signs of sanctification, but also bring it about. It is inevitable that they bring spiritual gifts in sensuous form, because of the sensuous nature of man. The res sensibiles are the matter, the words of institution the form of the sacraments. Contrary to the Franciscan view that the sacraments are mere symbols whose efficacy God accompanies with a directly following creative act in the soul, St. Thomas holds it not unfit to agree with Hugo of St. Victor that "a sacrament contains grace", or to teach that they "cause grace".
St. Thomas attempts to remove the difficulty of a sensuous thing producing a creative effect, by distinguishing between the causa principalis et instrumentalis. God, as the principal cause, works through the sensuous thing as the means ordained by him for his end. "Just as instrumental power is acquired by the instrument from this, that it is moved by the principal agent, so also the sacrament obtains spiritual power from the benediction of Christ and the application of the minister to the use of the sacrament. There is spiritual power in the sacraments in so far as they have been ordained by God for a spiritual effect." This spiritual power remains in the sensuous thing until it has attained its purpose. At the same time, St. Thomas distinguished the gratia sacramentalis from the gratia virtutum et donorum, in that the former perfects the general essence and the powers of the soul, whilst the latter in particular brings to pass necessary spiritual effects for the Christian life. Later, this distinction was ignored.
In a single statement, the effect of the sacraments is to infuse justifying grace into men. That which Christ effects is achieved through the sacraments. Christ's humanity was the instrument for the operation of his divinity; the sacraments are the instruments through which this operation of Christ's humanity passes over to men. Christ's humanity served his divinity as instrumentum conjunctum, like the hand; the sacraments are instrumenta separata, like a staff; the former can use the latter, as the hand can use a staff. For a more detailed exposition, cf. Seeberg, ut sup., ii. 112 sqq.
Of St, Thomas's eschatology, according to the commentary on the Sentences, this is only a brief account. Everlasting blessedness consists in the vision of God; this vision consists not in an abstraction or in a mental image supernaturally produced, but the divine substance itself is beheld, and in such manner that God himself becomes immediately the form of the beholding intellect. God is the object of the vision and, at the same time, causes the vision. The perfection of the blessed also demands that the body be restored to the soul as something to be made perfect by it. Since blessedness consists in operatio, it is made more perfect in that the soul has a definite operatio with the body, although the peculiar act of blessedness (in other words, the vision of God) has nothing to do with the body.