Little is known of his philosophy; he wrote several poems of which only 108 lines have survived. He rejected the Greek polytheism, attacking the anthropomorphism of the Greek gods. He set forth the doctrine that there is only one ultimate reality, "One god, supreme among the gods and men, and not like mortals in body and mind". This being does not have sense organs like men's, but "the whole of him sees, the whole of him thinks, the whole of him hears". He sets in motion all things by the power of his mind; he always remains in the same place, moving not at all; he is omnipresent, not needing to move.
The true founder of the Eleatic school is not Xenophanes, but Parmenides of Elea (c.515 B.C.-c.450 B.C.). He is probably the most important philosopher before Socrates, being the first to focus on the central problem of metaphysics, the problem of being. He held that "thought and being are the same thing", and that "being is and it is impossible for it not to be; it is impossible that non-being is". That is, being exists and non-being does not exist. Being is ungenerated and indestructible. It cannot come into being out of what does not exist. Neither can it cease to be, since being is not non-being. Hence being is eternal. Change is impossible; for if anything changes, it ceases to be what it is and becomes what it is not; and since being cannot cease to be and non-being does not exist, change is impossible. And also movement is impossible; for if anything moves, it must occupy empty space where it was not; and since empty space is non-being and does not exist, movement is not possible. And there can be only one being. For if there were more than one being, these beings would have to be separated by empty space; since empty space is non-being and does not exist, they cannot be separated and thus they must occupy the same space; thus the many are one and being is one. Finally being is homogeneous, not having parts; for if it has parts, then it would be many; but since being is one, it has no parts and thus is it homogenous. From this Parmenides argued that being is finite, like the surface of a sphere it is "perfected on every side", equally distance from its center at every point. Parmenides' student, Mellisus of Samos (5th cent. B.C.), rejected his teacher's conclusion that being is finite; being must be spatially and temporally infinite. For if being is finite, then beyond it there would be empty space; but since empty space is nothing, non being, it does not exist and thus being occupies all space. Thus being is spatially infinite. Mellisus, like Parmenides, rejected a void or vacuum, empty space, as impossible and non-existent, "for what is empty is nothing. What is nothing cannot be". Zeno of Elea (c. 490-430 B.C.) was a firm adherent to Parmenides' ideas and tried to show that they are true by showing their opposites are impossible and absurd (reductiones ad absurdum). By this form of argument he attempted to show that pluralism (that being is many, not one), empty space, and motion are impossible.
Plotinus was born in Egypt and studied in Alexandria under Ammonius Saccas. After accompanying the emperior Gordian on a campaign in the East, he settled in Rome about 244, opening his own school. He wrote essays for his students about their philosophical discussions. His pupil, Porphyry, collected these and arranged them systematically into six Enneads [groups of nine], that are the major source of Plotinus' philosophy. Porphyry published the Enneads after A.D. 300 with an accompanying Life of Plotinus. Prophyry reports that Plotinus had a mystical experience of union with the divine on four occasions, which union is described in the Enneads and is one of classics of mysticism.
In the fourth and fifth Christian centuries, Neoplatonism provided the philosophical basis for pagan opposition to Christianity. Prophyry in addition to his numerious philosophical treatises, wrote a massive fifteen volume work, now lost, Against the Christians. The Roman emperor, Julian, in addition to decrees against Christians, wrote Against the Galileans, which can be reconstructed from Cyril of Alexandria's refutation of it. In addition to providing the philosophical opposition, Neoplatonism provided the philosophical framework for the thought of several Christian theologians: Gregory of Nyssa, Victorius, Ambrose, Augustine, and later Dionysius the Pseudo-Areopagite. The influence of Neoplatonism continued in both the Western and Eastern churches. Even in the twentieth century elements of it has appeared in the thought of Paul Tillich.
The Catholic reaction to this form of puritanism is illustrated in the six (later expanded to seven) book which Optatus, bishop of Milevum, wrote in A.D. 366 or 367 to refute the Donatist leader and publicist, Parmenianus.
Augustine echoes and deepens these thoughts of Optatus' in his prolonged controversy with the Donatists. According to Augustine, the Church is the kingdom of Christ, His mystical body and His bride, the mother of Christians. There is no salvation apart from it; schismatics can have the faith and sacraments, but cannot put them to a profitable use since the Holy Spirit is only bestowed in the Church; in this he differed from his admired master, Cyprian, preferring the traditional Western doctrine that the sacraments are valid even if administered outside the Church. Augustine identified the Church with the church of his day, with its hierarchy and sacraments, and with its center at Rome. Its catholicity consists partly in its claim to teach the whole truth and not selected fragments of it, but evenmore, in its world-wide extension. This latter characteristic marks it off from the sects, each of which flourishes in a particular locality. Not that the Church, on Augustine's view, is to be confined to the universal, empirical society visible at an one time. It includes in its ranks not only present-day Christians, but all who have believed in Christ in the past and will do so in the future. This consisted in drawing a careful distinction between the essential Church, composed of those who genuinely belong to Christ, and the outward or empirical Church. With his Platonic background of thought this distinction came easily to him, for the contrast between the perfect essence, eternal and transcending sensation, and its imperfect phenomenal embodiment was always hovering before his mind. From this point of view, within the visible church there are those who belong to the essential Church, and the rest, that is to say, sinners, may seem to be within the Church, but they have no part in "the invisible union of love". They are inside the house, but remain alien to its intimate fabric. They belong to the catholicae ecclesiae communio and enjoy the communio sacramentorum, but it is the righteous who constitute "the congregation and society of saints", the "holy Church" in the strict sense of the word. Thus Augustine's solution of this persistent problem was to argue that the authentic bride of Christ really does consist, as the Donatists claimed, exclusively of good and pious men, but that this "invisible fellowship of love" is only to be found in the historical Catholic Church, within whose boundaries saints and sinners meanwhile gather together in a "mixed communion". Augustine then stressed that the church's purity was eschatological, incapable of present realization. The error of the Donatists was to make a crude institutional division between them, whereas the precedent of Israel showed that the division was a spiritual one and that God intended the two types of men to exist side by side in the world. Later as he worked out his doctrine of predestination, he was led to introduce a refinement on this distinction between the visible and invisible Church. He came to see that the true members of the Church could only be "the fixed number of the elect" known to God alone.
Augustine starting point is a glowing picture of human nature as it was created by God. Adam as he was created was immune from physical ills and had surpassing intellectual gifts; he was in a state of righteousness, illumination and beatitude. Immortality was within his grasp if only he continued to feed upon the Tree of Life. The freedom that he possessed, not in the sense of the inability to sin (the non posse peccare which Augustine believed to be the true freedom enjoyed in heaven by the blessed), but of the ability not to sin (posse non peccare). And his will was good, that is, devoted to carrying out God's commands, for God endowed it with a settled inclination to virtue. So his body was subject to his soul, his carnal desires to his will, and his will to God. Nevertheless, he fell, as the Bible records. Augustine considered that that was the fault of Adam himself. God could not be blamed, for He had given Adam every advantage; the one prohibition He imposed, not to eat of the tree of the knowledge of good and evil, was the opposite of burdensome, and his desires did not conflict with it. His only weakness was his creatureliness, which meant that he was changeable by nature and so liable to turn away from the transcendent good. Any blame must lie exclusively with his own will, which, though inclined towards the good, had the possibility, being free, of choosing wrongfully. When it did so, the latent ground of the act was pride, the desire to break away from his natural master, God, and be his own master. If there had not been this proud satisfaction with self in his soul, this craving to substitute self for God as the goal of his being, he would never have listened to the Tempter. And from this character of the first sin flows its heinousness. It was worse than any other conceivable sin in proportion as Adam was nobler than any other man and as the will which produced it was uniquely free. In fact, such was its gravity that it resulted in the ruin of the entire race, which became a massa damnata, sinful itself and propagating sinners. Augustine believed that the taint was propagated from parent to child by the physical act of generation, or rather as the result of the carnal excitement which accompanied it and was present, he observed, in sexual intercourse even of baptized persons. Augustine was divided between the traducianist theory and various forms of the creationist theory of the soul's origin. If the former is right, original sin passes to us directly from our parents; if the latter, the freshly created soul becomes soiled as it enters the body. There are two aspects in Augustine's explanation of the nature of original sin. In the first place the essence of original sin consists in our participation in, and co-responsibility for Adam's perverse choice. We were one with him when he made it and thus we willed it in and with him. As he expressed it, "In the misdirected choice of that one man all sinned in him, since all were that one man, from whom on that account they all severally derive original sin". Sin is a matter of the will, and "all sinned in Adam on that occasion, for all were already identical with him in that nature of his which was endowed with the capacity to generate them". To the objection that, if sin lies in the will, infants must be exempt from original sin since they cannot will freely. His answer is that there is nothing absurd in speaking of their original sin as voluntary, derived as it is from the free act of their first parent. As a result, while drawing a distinction between the guilt (reatus) of original sin and the evil it inflicts on our nature, he sees nothing incongruous in saddling us with both. Indeed, it is precisely this guilt, he argues, that baptism was designed to remove. Secondly, as a consequence of Adam's sin which is ours too, human nature has been terribly scarred and vitiated. Augustine does not advocate a doctrine of "total depravity", according to which the image of God has been totally obliterated in us. Even though grievously altered, fallen man remains noble: "the spark, as it were, of reason in virtue of which he was made in God's likeness has not been completely extinguished". But the corruption has gone far enough. The most obvious symptom of it, apart from the general misery of man's existence, is his enslavement to ignorance, concupiscence and death. In Augustine's vocabulary concupiscence stands, in a general way, for every inclination making man turn from God to find satisfaction in material things which are intrinsically evanescent. Far the most violent, persistent and widespread of these, is in his opinion, sexual desire, and for practical purposes he identifies concupiscence with it. As a by-product of our fall in Adam, we have lost that liberty (libertas) which he enjoyed, that is, of being able to avoid sin and do good. Henceforth we cannot avoid sin without God's grace, and without an even more special grace we cannot accomplish the good. Augustine does not intend to convey by this that we have been deprived of free will (liberum arbitrium) itself. While we retain our free will intact, the sole use to which we in our unregenerate state put it is to do wrong. In this sense he can speak of "a cruel necessity of sinning" resting upon the human race. That is, it is not possible for unregenerate man not to sin (non posse non paccare).
With this pessimistic view of man's situation, for Augustine grace was an absolute necessity: "without God's help we cannot by free will overcome the temptations of this life". The letter of the law can only kill unless we have the life-giving Spirit to enable us to carry out it prescriptions. And grace cannot be restricted to the purely external aids which the Pelagians considered it to be. Before man can even begin to aspire to what is good, God's grace must be at work within us. Grace is therefore "an internal and secret power, wonderful and ineffable", by which God operates in men's hearts. Augustine distinguishes four kinds of grace.
The Council of Carthage (A.D. 418), as confirmed by Pope Zositmus in his Epistula tractoria, outlawed Pelagianism in unambiguous terms. The main points that were insisted upon were
Augustine's exposition of the Trinitarian orthodoxy begins with the conception of God as absolute being, simple and indivisible, transcending the categories. In contrast to the tradition which made the Father its starting point, Augustine begins with the divine nature Itself. It is this simple, immutable nature or essence (he prefers "essence" to "substance", for the latter suggests a subject with attributes, whereas God for Augustine is identical with His attributes) which is Trinity. The unity of the Trinity is thus set in the foreground, subordinationism of every kind being rigorously excluded. Whatever is affirmed of God is affirmed equally of each of the three Persons. Since it is one and the same substance which constitutes each of Them, "not only is the Father not greater than the Son in respect of divinity, but Father and Son together are not greater than the Holy Spirit, and no single Person of the Three is less than the Trinity Itself".
Several corollaries follows from this emphasis on the oneness of the divine nature.
The Persons of the Godhead are identical considered as the divine substance, the Father is distinguished as Father because He begets the Son, and the Son is distinguished as Son because He is begotten. The Spirit, similarly, is distinguished from Father and Son inasmuch as He is "bestowed" by Them; He is Their "common gift" (donum), being a kind of communion of Father and Son , or else the love which They together pour into our hearts. Thus the question arises: what in fact are the Three. Augustine recognizes that they are traditionally designated Persons, but he is clearly unhappy about the term; probably it conveyed the suggestion of separate individuals to him. If in the end he consents to adopt the current usage, it is because of the necessity of affirming the distinction of the Three against Modalism ("the formula 'three Persons' was employed, not so that that might be said, but so as to avoid having to say nothing at all"), and with a deep sense of the inadequacy of human language. His own positive theory was the original and, for the history of Western Trinitarianism, highly important one that the Three are real or subsistent relations. His motive in formulating it was to escape a cunning dilemma posed by Arian critics. Basing themselves on the Aristotelian scheme of categories, they contended that the distinction within the Godhead, if they existed, must be classified under the category either of substance or of accident. The latter was out of question, God having no accidents; the former led to the conclusion that the Three are independent substances. Augustine rejected both alternatives, pointing out that the concept of relation (ad aliquid relatio) still remains. The Three, he goes on to claim, are relations, as real and eternal as the factors of begetting, being begotten and proceeding (or being bestowed) within the Godhead which give rise to them. Father, Son and Spirit are thus relations in the sense that whatever each of Them is, He is in relation to one or both of the others.
Augustine attempts to explain what the procession of the Spirit is, or wherein it differs from the Son's generation. Augustine was certain that the Spirit is the mutual love of Father and Son (communem qua invicem se diligunt pater et filius caritatem), the consubstantial bond which unites Them. His consistent teaching was that He is the Spirit of both alike; as he put it, "The Holy Spirit is not the Spirit of one of Them, but of both". This he believed to be the clear teaching of the Scripture. Thus in relation to the Holy Spirit the Father and the Son form a single principle; and since the relation of both to Him is identical, and where there is no difference of relation Their operation is inseparable. Hence Augustine, more clearly than any of the Western fathers before him, taught the doctrine of the double precession of the Spirit from the Father and the Son (filioque). Answering the objection that since both the Son and the Spirit derive from the Father there should be two Sons, he states, "The Son is from the Father, the Spirit also is from the Father. But the former is begotten, the latter proceeds. So the former is Son of the Father from Whom He is begotten, but the latter is the Spirit of both since He proceeds from both. ... The Father is the author of the Spirit's procession because He begot such a Son, and in begetting Him made Him also the source from which the Spirit proceeds." Augustine warns us that it should not be inferred that the Spirit has therefore two sources or principles; on the contrary, the action of the Father and Son in bestowing the Spirit is common, as is the action of all three Persons in creation. Further, despite the double procession, the Father remains the primordial source, inasmuch as it is He from Whom the Son derives His capacity to bestow the Spirit.
According to Augustine, there are analogies or "vestiges" of the Trinity everywhere, for in so far as creatures exist at all they exist by participating in the ideas of God; hence everything must reflect, however faintly, the Trinity Which created it. The function of these, is not so much to demonstrate that God is Trinity, but as to deepen our understanding of the mystery of the absolute oneness and yet real distinction of the Three. But for the veritable image, man should look primarily into himself, for the Scripture presents God as saying, "Let us [that is, the Three] make man in our image and our likeness". Even the outer man, that is, man considered in his sensible nature, offers "a kind of resemblance to the Trinity". The process of perception, for example, yields three distinct elements which are at the same time closely united, and of which the first in a sense begets the second while the third binds the other two together, that is, the external object (res quam videmus), the mind's sensible representation of it (visio), and the intention or act of focusing the mind (intentio; voluntas; intentio voluntatis). However, for the actual image of the Triune Godhead we should look to the inner man, or soul, and in the inner man to his rational nature, or mens, which is the loftiest and most God-like part of him. This is the mind's activity as directed upon itself or, better still, upon God. This analogy fascinated him all his life, so that in such an early work as the Confessions (397-398 A.D.) we find him pondering the triad of being, knowing and willing (esse, nosse, velle). In the De Trinitate he elaborates it at length in three successive stages, the resulting trinities being
Augustine has no illusions about the limitations of the analogies.
The second part (Books 6-10) Augustine takes up the philosophical problems of polytheism and Platonism. He demonstrates the internal inconsistencies of each, and shows the neither can provide the universal way of salvation found only in Christ. In this part Augustine so thoroughly and forcefully condemned the cult of the gods that paganism has never since been a serious religious option in Western society.
In the third part (Books 11-14) Augustine turns from attack to defense. He starts with the Scriptures, claiming that they are the only basis of all Christian thought. His method is clearly philosophical not exegetical, and his use of the Scriptures is open to question, although his general argument is clear enough. The Bible teaches that God created the world and it is essentially good, not evil as the Platonist maintained. Augustine denies the cyclical view of history, according to which the world would keep repeating itself. Augustine argues for a linear view of history with a specific goal in the second coming of Christ. He explains the origin of sin as the diobedinece of the first man, Adam, and that only the Creator has the power undo its effects. Augustine argues that human history must be understood as the struggle between good and evil, not in the abstract, but as two cities or kingdoms which belong respecitively to God and to Satan. These themes are developed throughout his The City of God, where Augustine sets forth a philosophy of history. It becomes a review of Roman and Christian history, interpreted theologically and thus eschatologically, through the entangled earthly fortunes of two "cities" created by two conflicting loves. Since the fall of man, "two cities have been formed by two loves: the earthly by love of self, even to the contempt of God; the heavenly by the love of God even to the contempt of self." (14:28) These had their representatives in Cain and Abel.
In part four (Books 15-18) the history of the world as given in the Bible is recounted. Starting with story of Cain and Abel Augustine reveiws biblical history showing the development of the two cities. In Book 18 Augustine develops the theme of the two cities much further. Jerusalem and Babylon are symbolic representations, and references to them in Scripture are usually typological in intent. According to Augustine, man is attracted in life by two loves - the love of God and the love of self - and human history results from their tension. By choice each man is a citizen of one of two cities, the City of God, or the City of Man, Jerusalem or Babylon. While the City of God is exempt from destruction, the City of Man is not. In the City of Man all of the risks of contingent being are present. Between these two alternatives there is a third - Rome as shaped by Jerusalem, the state shaped by the values of the City of God. To some extent, this is a relevant ideal, and was partially shaped by Augustine. But his clearest view is that the values of the City of God, held by the Church, are superior to those of civil society: the Church then is superior to the state. Later generations were to interpret the two cities as Church and state, but that interpretation was never Augustine's teaching.
Augustine divides human history into four periods: paradise before the fall, the world after the fall, the period of the law, and the period in which we now exist - after the first coming of Christ - a period of grace. Then follows the second coming of Christ and the final judgment. Augustine did not believe that there would be a literal thousand year reign of Christ on earth before the final judgment.
In part five (Books 19-22) Augustine concludes his work with a lengthy discussion of man's destiny. He interprets all human striving as a desire for peace, which can only be found in Christ. He discusses the last judgment and defends the concept of eternal punishment in hell. He then concludes with a discussion of the resurrection and eternal life, which he interprets as the mystical vision of God, the supreme fulfilment of the Christian's spiritual pilgrimage.