It is very difficult to summarize a book of dense summaries. Hopefully this post is not too abbreviated to be helpful.
Preface: As it is at the very least unhistorical to jump from Greece to the Renaissance, Carre sets out to examine the investigations of 4 medieval thinkers on the status of general ideas, or universals. It is in this central debate of the medieval era that one finds the roots of modern ideas about reason and knowledge.
I. St. Augustine: 1) During a brief biography, Carre apologizes for Augustine's "lack of rigor" due to the fact that he belonged to an "intellectual period in decay." (?) In any event, Aug. is said to be unique in arguing that, even after one has faith, one must toil for understanding, which, if sought correctly, is a kind of eternal wisdom and not a temporal knowledge. It is through Aug. that Greek philosophy is united with Catholic theology.
2) Augustine understood the pursuit of wisdom as a path of ascent from the changeable to the unchangeable. Although he posits different conceptions of the stages of ascent in different texts, in all he is concerned with showing the progressive stability and goodness of each stage. We ultimately arrive at an absolute, timeless reality, provided we are not distracted by the world of impermanence.
3) To combat Pyrrhonic skepticism, Augustine grounded knowledge in two arguments: 1) Si fallor sum: even when we doubt, we are aware of ourselves as existing in a state of doubt. The mind knows itself immediately as incorporeal, outside of space and time; it knows itself indirectly through the senses. (We should remember from the Confessions that knowledge for Aug. is anamnesis, i.e. an uncovering of what is already there). 2) Even skepticism assumes the principle of non-contradiction, which we can take as evidence of the moral law within. Unlike the Greeks, Aug. is concerned with the law within and not so much the outside world. But it is crucial that the inside is not just a "subjective" realm; it is the truth, more so than knowledge gained from the senses. There is Scientia (empirical knowledge) and there is Sapentia (eternal wisdom, intelligibiles res); only the latter is necessary, immutable and real.
4) The (active) mind is a wholly different substance from the (passive) body. Though we immediately think of Descartes, Carre asks us to remember that Aug.'s view comes directly from the Neoplatonic view of the mind as vehicle of spiritual order, though the results are somewhat similar.
The question arises, if the body is completely passive, how do we sense? Aug. argues that when the body's movement is impeded, the mind feels unpleasantness, and vice versa. The total effect of the various ways in which the impingements upon the body appear to the mind is called sensation.
Another question: if sensations are made by the mind, how do we know they correspond to reality? Aug. doesn't worry about this dilemma, and says, rather interestingly, that "the question of objectivity does not arise in relation to sensibilia" (15). Only judgments, not sensations, can be true or false (a vaguely Heideggerian move...the world is there for us in a way such that it can't really be questioned).
Next, Aug. posits non-sensory perceptions that result from an interior sense more primary than the 5 senses. He gets very close to Kant when he talks about spatiality being implied in perception.
And finally, whence general ideas? They are not derived from particulars. The whole notion of Aristotelian abstraction is foreign to him, as it is not the body and physical world that act upon the mind, but the mind that acts upon the body and the world. General ideas are independent of sensible origin, come from above and are real. In scientia, we apply our reason, itself "in contact with the intelligible and immutable truth," to "handle and direct the inferior things" (18).
5) Although Aug. is often taken to disparage all questions of science, he maintains a limited respect for investigations into the natural order in so far as it is understood to be an external reflection of the immutable truth. Investigation for its own sake is nothing but vain curiosity.
One of Aug.'s more interesting ideas about nature is that of rationes seminales, seminal reasons. This concept is born from the contradiction in Genesis between "God created everything at once" and the clear temporal progression that is later described. Aug. says that God created everything at once in potentia, and that each creature evolves according to the "germs of life" appropriated to it. We cannot create anything new. At most, we can facilitate the development of new creation for which God is alone responsible.
6) Unmistakably, the intellectual cognition of eternal things is to be preferred to scientia. Augustine points to mathematics to evidence the existence of immutable forms. Though number is absolutely distinct from its empirical instantiation, mathematical propositions are formal characteristics of the perceived world. Ideas are not just intelligible systems standing over and against empirical reality but are necessary to properly understand it. Moreover, because our minds are feeble, the mutable in its imperfection aids us in our pursuit of the immutable. In art, for instance, we contemplate the natural world in its unchangeable aspect (number).
But perhaps it seems that moral wisdom and mathematics are different things. No, says Aug., for the existence of form, of truth and of beauty as formal characteristics of the world evidence a supreme form and an absolute knowledge.
Aug. often talks about the "light" by which the mind is illuminated. In some of his texts, the light is truth, beauty and goodness. The mind is understood here in its active sense. In others, the light is God, and the mind is passive. Together, the light metaphors teach that "the principles which guide experience are realities which the mind is led to accept" and at the same time "that the meaning of these realities is not finally to be appreciated by purely intellectual approach" (29).
7) For our purposes, Aug.'s key idea is that universal forms constitute the nature of things. Oh, and Aug. was very influential.
II. Peter Abaelard: 1) From Plato the idea was inherited that the criteria of the real include permanence, objectivity and coherence. The unities which underlie the world (logical principles and standards of perfection) are alone real. Carre explains the prevalence of realism in three ways: a) Since Augustine, realism had become associated with philosophy itself. b) Platonism fit very well with Christian doctrine. c) The dominance of social institutions over individuals fits with realism.
Then came Boethius, who introduced the world to Aristotle and wrote an important commentary on a Porphyry text that was itself a commentary on the Categories. Here the question of the status of universals was raised. For Aristotle, we find the truth by finding a thing's essence, what distinguished it as itself. This universal essence can only be defined in one way.
2) Boethius, explaining Aristotle, argued that universals a) are real features of sensible objects, b) are immaterial, having no existence apart from sensible things, and c) do not exist independently of bodies. His commentary raised no controversy for four centuries, until Roscelin of Compiegne (1050-1125) propounded a full-fledged nominalism: general ideas are merely names, and common nature is wholly subjective. Anselm responded that if one could not grasp the way in which many individual men were united in the real species man, one could not understand the trinity.
3) Brief biograhy of Abaelard, born in 1079. He changed careers many times, first dialectics, then theology, then a mix of both. He was always known for his ruthless methods of criticism. In a much anticipated debate with Bernard of Clairvaux, Abaelard was condemned to silence by the Pope. He died without having ever reached Rome to protest this decision.
4) Abaelard describes realism as the doctrine that says only universals are real, and that mere accidents differentiate species into particulars. Universals subsist naturally, but exist actually in physical expression. This real subsistence can only be apprehended by pure thought.
His criticism of extreme realism runs as follows: as particulars only differ with regard to their accidents, being one in substance, there is no real distinction between individuals; clear thinking becomes impossible. He charges realists with pantheism, as there cannot be for them an essential difference between divine and physical substance.
He then turns to less extreme versions, like William of Champeux's doctrine of indifference, which accepts a pluralist basis of knowledge: things that are distinct are the same not essentially but indifferently, in so far as they are not different. Abaelard says this theory tries to combine distinct and relative senses of the terms universal and particular and ends in a similar muddle.
Finally there is a third position which understands the universal as collectio, the real collection of things included within it. But this position is also confused: the universal is to the particular not as the whole is to the part.
The first part concludes: the cardinal point of realism is that particular instances are inessential features. Abaelard says no, the manifold variety of things is not an illusion. The difference between individuals cannot rest upon accidents.
5) Abaelard then turns to contemporary theories of universals, specifically Roscelin's extreme form of nominalism, which he rejects. His guiding question is: to what do universals refer? General terms cannot be merely words: man is the name given to individuals in virtue of an element which is common to many of them. But is that commonality part of the individual itself, in the mind, or both? Of course, no such entity man exists. Man is a status (a thing's nature in the order of things). He admits that there is some common nature in things that is then grouped together in the mind; Abaelard is thus something of a realist. But he expands upon this limited realism when he adds a third, intermediate layer to object and perceiver: the image. When we perceive a tower, we retain an image of it in our mind which is itself the object of thought, not the tower itself. A universal term is one that refers to a generalized, confused and composite image of a number of things (a position Berkeley would later come to refute).
6) Abaelard now turns to answer Porphyry's three questions: a) Are universals real entities existing independently of the mind? Yes, they refer to reality, but in a certain sense exist only for thought, as they embrace an abstract view of concrete things. b) Are universals material or immaterial? They refer to real discrete objects and are thus material but conceive of these real objects immaterially. c) Are universals found in sensible objects or apart from them? A universal term does not point to any sensible object but because they are elicited from them, they may be described as being in them. And then his own question d) Could universals exist without particulars? No. If that were the case, there would be no many to which a universal could refer.
Universals are neither words nor things but sermones, concepts. Universal man is not that to which individual men owe their being, but neither is it a mere word. The activity of thought isolates common features, which are are genuine aspects of objects perceived. The mind then breaks up the complex detail of the concrete and refashions it into objects of thought. Sense and thought cooperate in all experience; when the mind perceives, general ideas are already present. Abaelard follows Augustine in saying that Ideas are patterns in the divine mind, but his theory of knowledge is Aristotelian.
7) Abaelard is often described as a conceptualist, but Carre tells us that this title does not suit him. Conceptualism states that the common element of a universal is abstracted arbitrarily, but Abaelard clearly admits a minimum of realism and does not believe that universals can be pure inventions of the mind. In fact, Abaelard is an early expression of the moderate realism developed later by Aquinas.
III. St. Thomas Aquinas: 1) In the 12th century, the west was introduced to the works of Aristotle through Avicenna and Averroes. Aquinas, who rejected both the Neoplatonism of Bonaventure on the one hand and Averroes/Avicenna on the other, melded Aristotle and Christianity with "Euclidean clarity."
2) Unless we understand Aquinas' philosophy of being, it is impossible to make sense of his epistemology. Following Aristotle, reality is absolute and unchanging. It is an end, to which all aspects of the universe point teleologically. Everything in existence is both potentia, in a state of imperfection that points to something higher, and act, or completion/stability/perfection. The more self-determined a thing is, the more it is pure act.
Potency and act are co-dependent. Matter is a passive potency, and form is the realization of matter in definite structures (act). Similarly, body is a passive potency dependent upon the act of mind. All things are related through potency and act. For instance, bricks may be potency in relation to the house they comprise, but are act when viewed from the perspective of the making of the brick. Growth, ethics and aesthetics are ideas that only make sense when we understand the relation of act and potency.
The essence of an object, a universal, is that in virtue of which it is what it is. But we may know the essence of something without it being something that exists. Existence is the concrete form that essence takes, and is thus act in relation to essence's potency.
3) Aquinas wants to show that objects of knowledge are material and not simply psychic entities, but also that an identity of subject and object is fundamental to experience. External objects must be shown to unite with the mind without changing their nature.
There are two ways a thing can change under the influence of form: 1) through "natural change," where form is physically transferred (for instance, when we touch something hot) and 2) through "mental change" (for instance, when we perceive the color of a thing without it changing the color of our eye). Here the mind reaches out to the object and renders it communicable in terms of likeness. Knowledge consists in the degree to which that likeness represents the object (exact opp. of Kant). The more intimately an object is known, the more it is one with the knower. The object is thus different from the subject but also a phase of its self-actualization.
4) Like Augustine, Aquinas holds that mind is incorporeal and wholly different from the body. But contra Augustine, he does not think the pure mind functions in experience. Man's mind is a mix of pure mind and body. Mind is in fact manifested at all levels of nature and finds its expression in varying degrees. The mind cannot function without its material expression, through which it comes closer to itself in act (sounds like Hegel). The mind is thus not related to the body as a pilot is to a ship; the body is a key component of knowledge because sensation is its root. Man's body and organs have very much to do with his essence.
Aquinas asserts that Plato's conception of the Ideas as creative substances is inadmissible in Christianity. But we can admit Aug.'s revision of Plato (divine Ideas) by saying that the human mind knows all things through eternal reasons because all intelligent experience is derived from God. But we have to add that there is only intelligible content to knowledge because of sensible experience. The body is not a hindrance to the mind. Only through it can the human mind gain knowledge.
5) Though the human mind has many potencies, mind is in essence act and thus constitutes a unity. Perception requires a general sensibility that is made explicit in knowledge. Sensation is the realization, or act, of the sense-organ; sensory elements are secondary. Thus, we know objects directly, and their signs indirectly. For instance, we know the hammer directly. That it has certain features we know only in reflection: "sensible images enter into consciousness when they are stripped of their sensibility" (Heidegger?).
Undeniably, there are two orders discovered in knowledge, that of thought and perception. But if the two are divided from each other, knowledge becomes impossible. Carre explains, quite succinctly, that "universality is indeed the characteristic of thought, but the objects of thought need not exist as universals" (85).
6) The active intellect (not part of the divine mind, contra Aug.) is uniquely human: its function is to make general, and thus intelligible, sensory particulars. Aquinas compares the active intellect to a light, which illuminates external reality and makes it understandable. Without its work of abstraction, we would not be able to know any particularity. The species which is abstracted serves as an "ideal intermediary which unites object and subject in an act of consciousness." The species is thus not an object but a means of thinking. Knowledge is concerned directly with reality, but it can grasp that reality only be freeing the intelligible form implicit in the particular. Truth is the conformity of the mind with the form pervading the object as mediated by the species.
7) Carre turns to an important section of the Summa where Aquinas differentiates his position from sensationalism on the one hand and Augustinian idealism on the other. There are three views on the matter of knowledge: a) Democritus: all knowledge can be reduced to impressions of sensible entities on the mind. b) Plato: thought is an immaterial power distinct from sensation, and it alone is the foundation of knowledge. c) Aristotle: a third way - sensation is not an activity of the mind alone but a complex of mind-body. Thought depends on sensation, but images themselves cannot affect the mind. They only become intelligible with the aid of the active intellect. This final position is also Aquinas's.
8) Though his interest in scientific matters was meagre, Aquinas refuted those who wished to attribute the cause of everything to God and belittle the role of natural causation. If God alone causes everything, then we would not see the effects that we do in nature. This is not to say that God is not present in the work: he is like a craftsmen with his tools. Nature is Gods instrument. Aquinas was fighting a resurgence of a Manichaen worldview that deemed matter evil. Divine goodness is the goal of all material things.
9) In sum, the progress of thought is the effort to realize the ideal unity which is the common ground of mind and objects. The mind apprehends universals, which are found in the particular and never apart from it, directly, and particulars indirectly. Because the active intellect makes sensory data intelligible by means of these universals, we cannot attain the essence of things directly, for we are dependent on perception, which compels our thought to be discursive. The modes of thought by which the mind understands the world are different from the mode of being in which nature itself exists.
IV. William of Ockham: 1) Nominalism was a sign of the disintegration of medieval philosophy. Its exposition by William of Ockham was seen as unorthodox and dangerous, and this led him into an intensely polemical life. He succumbed to the plague in 1349.
2) Aquinas died in 1274, and the remainder of the century was spent debating his ideas. A reactionary Augustinian movement broke out, and Ockham was born into this polemic. He was also the heir to Peter Abaelard and Petrus Hispanus, from whom he took the basic idea that the generality in our thinking is essentially subjective.
3) Ockham is most well-known for his razor, a rule of economy which was directed primarily against realism. For Ockham, universals are convenient mental fictions. He argues that if the universal "humanity" were indeed an entity distinct from a particular individual and at the same time part of its essence, it would be in a number of different places at the same time and thus be contradictory.
His own theory of knowledge runs something like this: the basis of all experience is intuition. Intuited knowledge is clear and certain, as opposed to abstract knowledge, which is by comparison doubtful and confused. Only intuition, a combination of sensory and intellectual factors, can produce an object, and it alone is reliable. We know reality directly, not through an intermediary like species intelligibilis.
Strangely, Ockham follows Augustine in saying that the knowledge we gain from our inner senses is more immediate than that of the outer senses, thus introducing some hesitation about his proposed certainty of the knowledge of objects. Ockham expands the category of empiricism to include many rationalist elements.
4) Ockham rejected extreme nominalism; for him, there is some kind of reality attached to general notions. Universals have a logical status of existence. A few different concepts are needed to understand his view of universals: first, he classifies the contents of thought under the heading of first and second intentions. First intentions are primary experiences, direct intuitions of things. Second intentions are signs of first intentions. He then introduces another concept, the suppositio, or substitutions, of which there are three: a) material: grammatical symbols (nouns, adjectives, etc.), b) personal: man for particular man, c) simple: man in general, refers only to concepts, not realities. Ockham is here distinguishing between real and logical meaning, between assertions about forms of discourse and assertions about things. Knowledge of reality is of a different order than logical knowledge.
Universals, then, are second intentions of the simple order. They refer to terms, not things. All previous difficulties with universals spring from the confused attempt to make them both singular and plural at the same time.
Ockham's challenge to theology is significant: for him, it is absurd to think, with Aug., that Ideas are things in the divine mind. Ockham would later come to change his view, saying that universals are not logical content formed by the mind but qualities of the mind itself.
5) Ockham's revolution can be summarized quite succinctly: existence is composed of individual items in various relations. No universal essences are required to be expressed in order for matter to become individual.
6) The relation between the provinces of faith and reason has always been a problem. Ockham denied that any of the central beliefs of of religion can be logically demonstrated. Conclusive philosophical knowledge of God cannot be attained. Scholasticism, which was inspired by the belief in the rational unity of philosophy and theology, was ruined.
7) The successors of Ockham, mainly British (Locke, Berkeley, etc.), developed his sceptical treatment of knowledge. Realism influenced Descartes and the rationalists. The end.