The teachings of Averroes became the basis for a whole school of philosophers, represented first by the Faculty of Arts at the University of Paris, among whom the most prominent was Siger of Brabant. No philosophy was more often condemned in the Middle Ages by church leaders and councils than Averroism. It was condemned in A.D. 1209, 1215, 1240, 1270, and 1277. It was condemned for its teachings that held that matter is eternal (God and the world are co-eternal), the absence of personal immortality (the numerical identity of the intellect of all men), and the doctrine of double truth (that a proposition may be true in philosophy and false in theology). Aquinas, relying on the translation of Aristotle by William of Moerbecke, criticized Averroes' interpretation of Aristotle. On the first point, Thomas argued that there is no philosophical proof, either for the co-eternity of God and the world or against it; but the creation of the world is an article of the faith. On the second point, Thomas argued that the unity of the intellect of all men should be rejected, since it is incompatible with the true concept of person and with personal immortality. On the third point, it is doubtful whether Averroes himself held to the double truth theory; but it was taught by the Latin Averrosts, who, not withstanding the opposition of the Roman Church and the Thomistic philosophers, gained great influence and soon dominated many universities, especially in Italy at the University of Padua. Thomas and his followers were convinced they had interpreted Aristotle correctly and the Averroists had misinterpreted Aristotle; Aristotle did not teach the double truth theory. Truth is one, but there are two ways to discover it: by revelation of it in the Bible and by reason in the writings of Aristotle. Where the two are contrary to each other, the truths of revelation are to be accepted and the results of reason are to be modified to conform to the truths of revelation. From A.D. 1272 Aquinas taught at the University of Naples. He died on A.D. March 7, 1274 on the way to the Council of Lyons. Aquinas was canonized in A.D. 1326, made a Doctor of the Church in A.D. 1567, and Pope Leo XIII (Aetrni Patris) gave his philosophy official status in A.D. 1879.
At the heart of Aquinas' philosophy is his concept of being. And to understand being reason must use the principle of analogy to fix the appropriate meaning of the term. And since there are certain "transcendental" terms that go beyond any genus, and apply to everything that is, they are attributes of everything. These transcendentals are ens (being), res (thing), unum (unity), aliquid (distinction), verum (true), bonum (good). All beings (ens) are things (res) with unity (unum), distinguished (aliquid) from what is not themselves. And since all beings are what they are, in relation to knowledge, they are true (verum). And since all beings tend toward their ends or goals, they are good (bonum). When the principle of analogy is applied to these transcendentals, human reason can begin to understand, within limits, the nature of God. The kind of analogy to be used here is the analogy of proportionality; that is,
The properties of x The properties of y are to as are to x's being y's beingBy the use of this analogy of proportionality human reason can begin to understand how God's being exceeds any other being by comparing the properties of being. Since there are five properties of being, there are five ways to establish this comparison. From this comparison there arises the idea of a perfect being, God. God is perfect and unchanging being, utterly simple (without parts) and unitary, hence indestructible, absolute truth and goodness, not related to the world, yet everything in the world related to Him. Since the divine mind contains the archetypes of all things, simply in knowing Himself God is able to know at once all that is, was, or will be. In His self-knowledge all time is concentrated in an eternal moment, in a totum simul. Aquinas argued that while God is the primary cause of all things, there are secondary causes and among these secondary causes some are necessary and others are contingent. Thus free will is compatible with God's foreknowledge and God's causation of all things.
At the heart of Aquinas' ontology is the real distinction between essence and existence in all finite beings. Aristotle distinguished between actuality and potentiality, but he applied this to form and matter, not to the order of being. Aquinas argued that only God is pure being, pure actuality (actus purus), with no potentiality whatsoever. That is, God's being is from Himself, not from another. He has aseitas (from Latin a se "being for oneself" hence, "self-caused" and "uncaused") in contrast to those beings that have derived their being from another (ab alio). Thus God is necessary being, because God's essence, essentia, (what He is) and His existence, esse, (that He is) are identical, hence He cannot fail to exist. In finite creatures, their essence (what they are) is separate from their existence (that they are). Aristotle does not make or use this distinction between essence and existence; Aquinas introduced this distinction because it allowed him to explain the difference between God and the angels. Angels are pure forms like God, but their essence and existence are not identical, unlike God whose essence and existence are identical. According to Exodus 3:14 God revealed Himself as the "I am". Aquinas interpreted this to mean that God alone is Being (I-Am-ness). Everything else has being. God's essence is identical to His existence; that is, it is of His essence that He exists. Thus God is a necessary being, not a contingent being like everything else; He cannot not exist. Neither can God change, since He is without potentiality to be anything other than he is. Likewise, God is eternal, since time implies a change from a before to an after. But as the I-am, God has no before and no after. God is also simple (indivisible), since He has not potential for division. And he is infinite, since pure act as such is unlimited, having no potentiality. Thus God is perfect Being.
Aquinas views reality as a hierarchy of being. Like Aristotle, he viewed the lowest level of reality as pure matter, without form, that is, prime matter, and the highest level as pure form, without matter, that is, God. God is regarded as pure actuality; prime matter, on the other hand, is viewed as pure potentiality. But Aquinas also held that since prime matter cannot exist by itself, it is dependent upon form for its existence in a concrete individual substance. Since form on the hand is not dependent upon anything for it to exist, it can exist from itself and indeed as pure form it must exist. But in Aquinas' view, prime matter does not exist; it has only the potentiality of existence. God, on the hand, does exist and cannot avoid existing. Like Aristotle, Aquinas held that between these extremes are to be found various levels of formed matter, the order of nature. Concrete individual substances are constituted of the abstract metaphysical elements of form and matter. Aristotle described substance as the union of form and matter. For example, man is the substantial union of form (mind or intellect) and matter (body). By contrast the Augustinians of the thirteenth century held that man is the union of two different substances, the intellectual soul and the material body; a man is a soul in a body. That is, there is gap between the intellectual soul and the material body, which for Bonaventure is the guarantee of the soul's spirituality and its immortality. Albert the Great somewhat closed the gap by asserting that there is one substantial form between the soul and the body, the form of corporeity. The soul, he tells us, can be viewed either in itself, as an intellectual substance, or as a form exercising the function of animating a body. The first view, defining the soul's very nature, he attributes to Plato; the second, describing one of its external and accidental functions, he attributes to Aristotle. Here Albert was simply following the Arabic philosopher Avicenna (A.D. 980-1037), who had already tried to reconcile the two Greek philosophers in this way. What neither Avicenna nor Albert could see by considering the soul as a substance in another substance such as the body, was the relation between them is purely extrinsic and accidental. Now this view did safeguard the independence of the soul from matter and its immortality; but it is difficult to see how, under these circumstances, man is anything more than an accidental aggregate of soul and body. Thomas Aquinas saw this problem. The view of the Aristotelians and Averroism, on the one hand, threatened the independence of the soul from the body and the immortality of the soul; but, on the other hand, the view of the Augustinians and the Avicennians threatened the unity of man.
Albert the Great considered soul and body as radically distinct because he thought of each as an essence which by definition differs from each other. The Avicennian world is composed of essences of this sort, each of which corresponds to a definition and includes only what is contained in its definition. Whatever is outside the definition is accidental to it. For example, when we define man as a rational animal, nothing is said about his individuality and universality; these are accidental to the essence of man as such, so that it can be individual in Peter and Paul and universal in the concept we form of it in our mind. In addition, although the definition of a thing tells us what it is, it does not say whether it exists or does not exist. That is, existence itself is not included in the essence of a thing but is accidental to it. This is true of everything except God, whose essence includes his existence. William of Auvergne (A.D. 1180-1249) adopted from Avicenna this view of the accidentality of existence and used it to explain the contingency of created being. For, he reasoned, if God is existence, all other things must receive existence as an accident of their essence. Existence, then, is given them as a gift and they are contingent in their very being. Albert the Great expressed this same view, but Aquinas transformed and used it for his own purposes.
Thomas Aquinas saw that solution to the problem of the unity of man was, not to consider essence as primary in the understanding of being, but existence. Instead of the world being composed of forms or essences, Aquinas viewed the world as consisting of individual acts of existing (esse). Existence has the primacy in the concept of being. The form of each being is that whereby it is what it is; it is the principle that specifies and determines it to be a certain kind of being. But in addition to form there is a further and ultimate act[uality] that makes it to be or to exist. This is the act of existing, which Thomas describes as "the actuality of all acts" and the "perfection of all perfections". It is the most profound in any being, its ontological nucleus, so to speak, the source of all its perfections and its intelligibilities.
The soul when looked at from the point of view of essence or nature, it appears deficient and in need of the body, for it is only a part of the complete essence of man. But from the point of view of existence, this is not true. As a substantial form the human soul has a complete act of existing (esse), and since it is a spiritual form, its act of existing is itself spiritual. When it informs the body it communicates to it that act of existing so that there is but one substantial existence of the whole composite. For Thomas, therefore, the unity of man does not consist in a combination or assemblage of various parts or substances, but in his act of existing. This is the reason that Aquinas denied the presence of several substantial forms in man. If a substantial form gives substantial existence, several forms of this kind would give man several existences and his unity as a substance would be destroyed.
Aquinas upheld this doctrine of being in the face of wide spread opposition from his contemporaries. On the one hand, philosophers like Siger of Brabant wished to return to the Aristotelianism of the Averroes. Siger reminded Thomas that Aristotle had written about form and matter and composition of the two in substances; he had never mentioned an esse distinct from them. On the other hand, he faced the opposition of those who admitted esse as a distinct principle of being but simply treated as an accident of essence. This had been the view of William of Auvergne and Albert the Great, who traced the concept to Avicenna. For Thomas this was still to view being as primarily essence or form and to reduce the role of existence to an accidental determination of essence. Thomas stood alone in his century and indeed in the whole Middle Ages for the doctrine of existential being.
It was the Arabian philosopher, Avicenna, who taught Scotus to conceive of being as an essence in an absolute state. But there was much in the Arabian's metaphysics of essence which as a Christian Scotus rejected. For one thing, according to Avicenna God is not a free creator; all things flow from Him in a definite hierarchy with all the rational necessity with which conclusions are drawn from premises. Now, necessity enters the Avicennian world - and the world of Greek and Arabian philosophy in general - precisely because it is a rational world of intelligible essences. For even though existences or facts are contingent, essences are necessarily what they are. The problem which Scotus faced was to reconcile the freedom of God and the contingency of created things with the fact that there are intelligible essences in the universe and ideas in the divine mind. His solution was to assert the transcendence of God as infinite being above all essences, and to teach a radical voluntarism according to which all things, even the divine knowledge, are subject to God's will. God's will is free respect to contingent things, so the reason for creation is God's free choice. At the same time there is necessity in God and this also is compatible with God's freedom. If a person chooses to hurl himself over a cliff and, while falling, necessarily continues to will that fall, freedom and necessity are compatible. This compatibility of necessity and freedom are also to be found in God. Although God is free, it does not follow that the will of God is arbitrary. Indeed, the divine intellect, perceiving what conforms with human nature, provides the content of the moral law; but the sanction of God's will forces us to regard its transgression, not as an irrationality merely, but as a sin. The will of man is essentially free, and indeed it would not have been possible for God to have created a rational will incapable of sin.
Ockham criticized the synthesis of the philosophical system of Aristotle with Christian theology that had been the fashioned by the thirteenth century schoolmen such as Thomas Aquinas. Franciscan scholars from Bonaventure to Duns Scotus tried to argue for the Christian faith by destroying Aristotle's philosophy. But all these thirteenth century systems depended upon the Realism, the view that all universals or general ideas have an independent reality. Ockham rejected this view of universals on the basis of a radical empiricism in which the basis of all knowledge is direct experience of individual things. Hence universals have no independent reality but are only names or mere vocal utterance. Reality is attributed only to particulars or individual things. This epistemological position is called nominalism (From Latin, nominalis, "belonging to a name") and was introduced by Porphyry (A.D. 233-304) who attributed it to Aristotle. Ockham taught that terms are only mental realities and are universals only insofar as the can stand for many things. This universality is purely functional and does not refer to a common essence possessed by many things outside the mind (ontological nominalism). Reality is just a collection of absolute singulars and therefore can not give scientific evidence for the existence of God. Faith, not Reason, is the basis of the knowledge of God. Ockham's philosophical methodology was based on the rule that "What can be done with fewer assumptions is done in vain with more" (the Principle of Pasimony, or "Ockham's Razor"). His philosophy was called the via moderna as opposed to the via antiqua of Aquinas. Nominalism was of great importance for the physical sciences since it suggested that natural phenomena could be investigated empirically. But to Ockham God was above all knowledge and therefore cannot be known by reason, as the Thomist taught or by illumination, as the Augustinians believed, but only by faith. Ockham was highly critical of the traditional arguments for the existence of God, but accepted the reality of God through revelation. Ockham was certain that God is all powerful and the free creator of the world. Like Scotus, he did not think this could be proved by natural reason; but he know it to be true by revelation received by Faith. The only question for Ockham was: Can the omnipotent and freedom of God and the contingency of the world be saved in the way Scotus tried to do it? Ockham was convinced that they could not; and essential reform would have to be made in Scotism and every trace of intelligible essence removed from it if the danger of necessitarianism was to be avoided. In his criticism of Scotism and other contemporary philosophers, his first aim was to eliminate common natures and universals from reality, and in doing so he proceeds as a logician. When terms are used in propositions they serve as substitutes for things. This function of terms standing for things in propositions the schoolmen called supposition (suppositio). Now, there are three ways in which a term can exercise this function.
If real being is thus radically individual, then what is a universal and what relation has it to things? A universal, for Ockham, is simply a sign which stands for many things. Now signs, he tells us, are either conventional and artificial, like written or spoken words, or natural, like the noise an animal makes to signify its feelings. There are universal artificial signs outside the mind, but on analysis they are found to be simply individual things whose signification is purely conventional. But within the mind there are the natural signs or terms that are universal concepts. Their signification is not conventional but natural, since they are produced in us in an obscure way by nature itself as likenesses of things. That is why the concepts of men are alike while their languages differ. As to the exact reality of these concepts, Ockham, after some hesitation, seems to have taken the view that they are simply our acts of understanding.
Ockham developed a theory of knowledge to support his shift from common natures to individual things. The primary object of the senses and intellect can no longer be common reality, as in Scotism, but individual things. Following the terminology of Duns Scotus, Ockham distinguished between intuitive and abstractive knowledge. Ockham held that intuitive knowledge is always concerned with a singular things as existing and present to the observer. Abstractive knowledge, on the other hand, tells us nothing about the existence or non-existence of things, but concerns abstract ideas or representations. For Ockham, all our knowledge begins with an intuition of the senses and this is followed by an intellectual intuition. Abstractive knowledge comes afterwards and depends on them. Ockham insisted as strongly on the primacy of the individual in knowledge as on its primacy in being.
Using the Principle of Parsimony, Ockham argued that the concept of being is univocal, predicable of God and of creatures in the same sense, even though the two may not be either substantially or accidentally alike. He understood the term "substance" in the sense of Aristotle's primary substance, that is, the individual subject of qualities, so that corporeal substance is the individual subject of sensible qualities. Ockham views matter not as pure potentiality, like Aristotle, but as body with spatially distinguishable parts. Ockham views form as the structure of the material parts. He finds efficient cause to be the most useful of Aristotle's causes, and thinks final cause a metaphor.
In this way Ockham attempted to rid theology and philosophy of Greco-Arabian necessitarianism. Scotus thought that he could do it and still retain the divine ideas and common natures with the necessity which they introduce into the world, if they are subordinated to the divine will. Ockham would not concede even this much. While keeping Scotus' voluntarism, he abolished universals from God's mind and common natures from things, with the result that he had nothing left but an omnipotent God governed by no law save that of contradiction, and a universe of individual things, no one of which has anything in common with any other. In such a universe God can act in a very arbitrary way. God can, if He wishes, make it good for us to hate Him. Hatred of God, theft, adultery are bad only because of the will of God, not for any intelligible reason. So also God can make a fire cool, just as easily as He makes it hot, for there is no necessary connection between cause and effect. The nominalist universe of Ockham is thus a world of fact rather than one of intelligible necessity, a world of things to be experienced rather than one of intelligible natures to be understood. Such a world would be of interest to the experimental sciences, but would be barren for philosophy which aims at understanding reality.
Nominalism was a success in the fourteenth century and its influence was felt in many of the theological developments of the later Middle Ages. Its influence is seen in the tendency of that period towards probabilism and skepticism in philosophy and especially in natural theology. At the same time it lead to a more biblical theology and to fideism and mysticism. Nominalism was also influential in the domains of logic and experimental sciences.