Plato lived from approximately 428-328 B.C. in ancient Greece.(1) He was the student of Socrates and teacher of Aristotle. St. Thomas Aquinas lived from A.D. 1224-1274, during the medieval period.(2) Although separated by more than fifteen hundred years, few philosophers have had as much impact on western thought as these two men. Both introduced revolutionary ideas in the study of reality and knowledge. The purpose of this article is to present and compare the metaphysics and epistemology of Plato and St. Thomas Aquinas.
It is not always easy to differentiate and neatly separate the metaphysics from the epistemology of both these giants of philosophy, yet an attempt will none the less be made, no doubt with some unavoidable overlap.
To talk about metaphysics is to talk about reality. What is the nature of reality? Plato's and St. Thomas Aquinas' views differed significantly on this topic. Both views will now be examined, then compared and contrasted.
According to Plato, nothing that we experience with our five senses is real. We perceive change all around us. The movements of the ocean, the growing of vegetation are all examples of change. According to David Stewart and H. Gene Blocker, Plato posited that reality is "unchanging, eternal, immaterial, and can be detected only by the intellect."(3) Plato referred to these realities as Ideas or Forms.
Plato speculated that since we have ideas of perfections that do not exist in the real world, they must exist somewhere. For instance, nowhere can we find a perfect line, since a perfect line has no width or height and is one dimensional. Well, since we've never seen a perfect line then the idea must exist somewhere outside of the world we interact with through our senses. The same is true for moral ideas. When we talk about something being just, we are comparing it to some absolute and perfect standard. So, where does this perfect standard exists? Again, it must exist somewhere because we are all aware of these concepts and how they compare to one another, but yet again, it does not exist here in the material world. If we can talk about some things being more just than others, we must be aware of perfect Justice. The same applies to beauty. We know that a rose is more beautiful than a pile of garbage. There is clearly an absolute standard of beauty of which we are all aware, yet Beauty in and of itself, we have never seen in this world.
Plato further pointed out that we talk about classes of things such as trees and dogs, but these terms do not refer to a particular object, but classes of objects. So then, what are we talking about when we talk about dogs? There must be something such as dogness that allows us to incorporate specific dogs into its domain. These classes are not something that we can perceive with the senses, but it must exist because it makes no sense to talk about nothing.(4)
Plato also talked about things that exist apart from man. There are things that exist that we merely discover, such as the laws of gravity, mathematics, and logic. One plus one would be two even if no humans existed, just as the law of gravity would continue to exists, as would the law of non contradiction. But man did not create these laws. So, where did they exist prior to man's understanding of them?
In order to explain this phenomenon, Plato introduced the idea of Forms. According to Plato, human beings participate in two separate worlds. One is the material world with which we are all familiar: we experience this world through our senses. It exists within space and time and contains particular objects such as trees, cats, and dogs. The other world is made up of immaterial and eternal essences that we apprehend with our minds. According to Plato, this world of the Forms is more real then the material world that we live in because our sensible world only contains copies of the real things which exist in the world of the Forms. Frederick Copleston states Plato's position this way: "Sensible things are copies or participations in these realities, but the latter abide in an unchanging heaven of their own, while sensible things are subject to change, in fact are always becoming and can never truly be said to be. The Ideas exists in their heaven in a state of isolation one from another, and apart from the mind of any Thinker."(5) Tom Howe puts it this way: "For Plato, the Ideas or Forms are distinct from sensible things. They are completely objective entities, not simply concepts in the mind, and they exist separately from sensible things, outside of space and time. Sensible things participate in the Forms either by sharing or imitation."(6)
A Form then is an eternal, unchangeable, and universal essence.(7) In Plato's theory, our material world contains imperfect copies of the ideal Forms which exist in the world of Forms. It is in this world of Forms that such things as perfect Justice, Truth, and Beauty exist. It also contains mathematical entities such as numbers and the perfect circle. According to Plato, this explains why we all have the concept of things such as perfect justice and a perfect circle even though we have never truly encountered one. We all know a circle when we see one. We can judge whether one object is closer to another to being a circle. Yet a perfect circle is something that does not exist in this world. By definition, "A circle is an enclosed line every point on which is equidistance from a given fixed point that is its center."(8) The problem is that all circles in real life are bounded by a line, and that line has a width. If the line has a width, then where along the line do we measure the distance to the center? A perfect circle would be bound by a line of zero width. No such thing exists in the real world. Yet, we all have the same concept of the perfect circle. This concept must exist somewhere. It clearly does not exist in the material world. Hence, according to Plato, this perfect idea exists in the world of Forms, and we all have access to this Form. These entities exist apart from man; but not just in the mind of man. They have existed eternally apart from man. These things would exist even if no human being ever existed. A perfect circle and mathematical entities exists whether or not there is a human who is aware of it.
To illustrate, when we see a dog we recognize it as a dog. This is despite the fact that there are various breeds, varying in size, shape, and color. Yet they all belong to the class we know as dog. What makes a dog a dog? If one were to include in the definition of a dog, an animal with four legs, then how is it that we instantly recognize a three legged dog as a dog? According to Plato, there is a Form known as dogness. It is the perfect Form of a dog. We recognize specific dogs as participants in the Form of dog. The same is true with catness and treeness as well as all objects of sensible reality.
There is debate about where or in what form these Forms actually exits. Copleston rejects the idea that the Forms themselves are sensible objects: "It is absurd to speak as though the Platonic Theory involved the assumption of an Ideal Man with length, breadth, depth, etc., existing in the heavenly place."(9) What is clear though, is that Plato believed that the way we actually come to know these Forms is through our pre-existence. According to Plato, Forms, matter, as well as our souls, are eternal. Plato maintained that we had direct knowledge of these Forms before our birth, but through the trauma of birth, we forgot them. Life is the process of trying to regain that knowledge that we lost during birth. His theory also involves the belief in the immortality of the soul and reincarnation. We are basically a soul that inhabits a body. Our body is a hindrance to true knowledge. When we die, we return to the realm of the Forms where we await another rebirth, and thus the process begins again.
But how do we get from Forms to matter? It appears that the Greeks had no concept of ex nihilo creation. Plato attributes this act to what he calls the Demiurge. The Demiurge used the pre-existing matter and formed the matter using the Forms to bring about sensible objects. Copleston writes, "Plato pictured the Demiurge as introducing order into the world and forming natural objects according to the model of the Ideas or Forms."(10) There is debate about what the Demiurge actually represents. Some suggest it is God, others, such as Copleston, believe it "is probably a symbolic figure representing the Reason that Plato certainly believed to be operative in the world."(11)
Plato believed that the universe existed hierarchically, in three major levels.(12) We can picture this in the form of a pyramid. At the base we have the world of particular things. This is the material world apprehended through our senses. Above the base, we have the world of Forms. This level is divided into the lower and higher Forms. Ronald Nash proposed that the lower Forms include Forms that can be imagined such as a perfect circle, and the higher Form includes concepts humans cannot change such as Forms of Truth, Beauty, and Justice.(13) At the height of the pyramid is the Good. The nature of Good is in debate. Ronald Nash believes there is a clear relationship between the Good and the Christian concept of God.(14)
The metaphysics of St. Thomas Aquinas differed significantly from that of Plato. This is evident in the distinction made by Thomas Aquinas between form and matter. Every material substance is composed of both form and matter. The form is the universal element that places an object in its class or species such as horse or chair. The individuated particular members of the class are found in the matter. For instance, John and Peter have the same form-the form of humanness. But they are two separate and distinct persons. John is 5 foot 6 while Peter is 6 foot 5. It is the matter that distinguishes between the two.
St. Thomas also made a clear distinction between act and potency. Act is that which exist, while potency is that which can be, or the capacity for being. To illustrate using our previous example, John as a man has actuality, that is he is actually a man. He is actually 5 foot 6, and weighs 160 lbs. But as a man, John also has a certain amount of potentiality. He can become seven feet tall, 350 lbs. As a man he can also be black, white, or any other color that man can potentially be. It is act which gives something whatever positive perfection it has, and potency which allows it to acquire a new act or perfection which it does not have at the moment.(15) It is through this concept that Aquinas addresses one of the main concerns of philosophers; that of how something changes yet retains its identity.
God alone is Pure Act, since He has no potential to become something else. Every contingent being, however, is a combination of act and potential. M. C. D'arcy writes of Aquinas' view: "every being which can change in any way, which needs the help of any other being to explain itself and to exits, must be composite. It is not pure act; it is act in so far as it is something actual and determinate, but it is relative non-being or potency in so far as it does not contain a necessity for existing with itself?"(16)
At the center of Aquinas' metaphysics is his distinction between essence and existence. Essence is what a being is. Existence is the act by which a being is.(17) Or to put it another way, the essence of something is its what-ness, while its existence is its is-ness.(18) What something is is its essence, the fact that it is is its existence. Under the philosophy of Aquinas these are distinct. But although distinct, there is a definite dependence on one another. One cannot exist without the other. Frederick Copleston writes "There is no essence without existence and no existence without essence; the two are created together, and if its existence ceases, the concrete essence ceases to be."(19) Copleston adds "Existence, then, is neither matter nor form; it is neither an essence nor part of an essence; it is the act by which the essence is or has being"(20) and "existence is not a state of the essence, but rather that which places the essence in a state of actuality."(21)
Not surprisingly, Aquinas' metaphysics begins with God. God is Being. In Him essence and existence are identical. He is necessary, essential, and without limitation. He is existence while everything else has existence. This existence is necessary if anything was to ever exist since for anything to be possible there must be something existing. And it must be of His nature to exist; non-existence is not possible for God. In everything else there is a distinction between essence and existence. God is the uncaused cause of everything else so His existence must be necessary. That is, He could not have not existed.
So, in the hierarchy of being we have God who is Pure Act in which essence and existence are identical. Below Him are angles: they posses no matter because they are spiritual beings; they are pure form; they have both essence (potency) and existence (act). Below the angles, we have man: his form is that of the rational soul and body; he also has both essence and existence. Below humans are things: they have no soul; they are purely material form; they also have both essence and existence.(22) As we can see, the composition of act and potentiality is found in every finite being.(23)
There also exist a relationship between form and matter, act and potential, and essence and existence. It is the form of something that determines or completes its essence, but the essence is actualized by its existence. The form is that which is, but existence is the act by which the form is.(24) Matter is described by St. Thomas as pure potentiality, while form is act.(25) Copleston writes "The cause of existence; it is act, distinct from the potentiality which it actualizes."(26)
Although there may seem to be many differences between Plato and St. Thomas Aquinas, they all stem from one primary difference; that is in the idea of existence.(27) All of the other differences between the two philosophers can be traced to this point. Plato was unaware of the nature of being from the point of view of existence. This resulted in significant consequences: one such consequence being the rejection of existence itself. Anton C. Pegis writes the following in reference to Plato's concept of Forms or Ideas: "The Ideas are models of material things whose materiality, they are yet powerless to cause, by so much Plato excludes from his explanation of reality the existential conditions of sensible beings. This dose not mean merely that Plato was unable to account for the existence of matter, or to include matter within the causality of the intelligible source of all being. The malady is much more deeper than this. Plato's flight from matter is bound to be a flight from existence; for a metaphysics which does not deal with being as a whole does not deal with being at all. Plato could not successfully exclude matter and becoming from the domain of being, however much he may have tried rather, by trying to exclude them, he excluded himself. And this is another way of saying the Platonic metaphysics of the Ideas is a metaphysics of flight from existence."(28)
Aquinas rejected Plato's notion that man is a soul simply using a body. This would lead us to the conclusion that the whole nature of man is in the soul. But we know that the soul alone is not a complete being. A man is only a man when he can not only think, but see, hear, imagine, and remember. These things are not possible by the soul alone. It requires a body. According to Aquinas, man is a composite being; he is composed of both body and soul. But instead of the Platonic view of man being a soul in a body, Aquinas maintains that it is the body that is in the soul, not the soul in the body.(29)
Plato's metaphysics concentrated on the Forms which were abstractions of real sensible objects. Aquinas contends that objects exist in and of themselves and that each object has actuality, that is, its own existence. Aquinas separated existence from essence; Plato made no such distinction. Consequently, Plato cannot explain the concept of change in his metaphysics. John, who at birth weighed 8 lbs, now at the age of 40, weighs 160 lbs. Yet, it is the same John. Through all of the physical changes there is an identity which is consistent. This is easily explained in Aquinas' system. Although weighing only 8 lbs at birth, John always had the potential to be 160 lbs. His potential was simply not actualized until he actually reached a weight of 160 lbs. The metaphysics of Plato simply cannot account for the fact that something that exist as perfect and unchanging in another immaterial world exist as something constantly changing in this world.
Another distinction between Plato and Aquinas is in the idea of Forms. Both adhered to some type of concept of Forms; however, their use of the term was different. According to Plato, Forms were universals that existed separately from their objects. Aquinas denied universal Forms or the concept of some unchanging, immutable, immaterial object apart from sensible reality. Aquinas believed that form and matter together formed objects in reality. But the form of Aquinas was an immanent form that assigns an object to its class. For instance, John is of the form humanness. The main distinction being that the form is part of John, not something out there.
But how then did Aquinas deal with the concept of absolutes such as Justice, mathematical concepts, logic, and laws that exist separate and apart from man? Aquinas dealt with these through God. It is God who instituted the laws of the universe as well as mathematics and logic. It is God who also instituted the absolute concepts of morality, such as Justice. This is addressed in Aquinas' concept of the Eternal Law, of which Ronald Nash writes "Eternal Law is the law of God that applies to all of creation. The eternal law includes both moral laws and the physical laws that govern all of nature. Eternal law is God's mind conceiving and determining everything that exists. Every other form of law flows from the eternal law."(30) Aquinas addressed the moral aspect of this in particular by writing "Infused virtue is caused in us by God without any action on our part."(31)
When we talk about epistemology we talk about knowledge. How do we come to gain knowledge? Plato and St. Thomas Aquinas had very different views on this topic. Both views will now be examined, then compared and contrasted.
According to Plato, the only true knowledge that exists is the knowledge of the Forms. His reasoning was that since we can only know the imperfect changing copies that we come into contact with through our senses, this is not knowledge of the thing itself, but only knowledge of the copy. The only way to acquire knowledge of the thing itself, the Form, is through reason.(32) The senses can never give us true knowledge, only opinion. For knowledge to be true, it must be infallible. But objects of perception are in a constant state of change. They never are, they are always becoming.(33) Copleston writes of Plato's view "true knowledge of sensible objects is unattainable, and-by implication-that true knowledge must be knowledge of the universal and abiding."(34)
Our senses can deceive us. For instance, looking a railroad tracks that seem to merge in the distance or seeing a mirage in the desert. In these examples our senses betray us. It is only by our reason that we can determine that railroad tracks do not merge or that a mirage is not real. Furthermore, Plato argues, "if knowledge is perception, then no man can be wiser than any other man, for each man is the best judge of his own sense perception.(35)
Plato's epistemology makes a distinction between opinion and knowledge. There are degrees of knowledge. He illustrated this using the concept of the "divided line." One is to image a horizontal line that separates the world of reason from the world of senses. The bottom portion of this line would refer to the visible world -the world we experience with our senses. According to Plato, knowledge attained here is merely opinion. This portion is further divided by a shorter horizontal line into two further sections: Conjecture and Belief. The lower part of these two is Conjecture. Conjecture refers to "first shadows, then appearances produced in water and in all close-grained, smooth, bright things, and everything of the sort."(36) In other words, Conjecture refers to the reflections of material objects. On the line above Conjecture we have Belief. Here we have the actual things of which Conjecture was only a foreshadow. For instance, if I am looking at a horse, this experience is Belief. I can actually experience, touch, and smell the horse. But if I am looking at a picture or reflection of the horse, the picture or reflection is relegated to Conjecture. It is only a shadow or image of the material thing.
Now on the top half of the long line, in the area of knowledge, we come to know things by reason, not experience. This realm is inhabited by that which is the true object of knowledge, the Forms, which are distinct from the objects that are sensed in the realm of opinion.(37) This section is also divided by a shorter horizontal line which divides Dialectic and Understanding. Understanding is relegated to the bottom of this shorter line. Understanding is where we find mathematical concepts. Copies of the actual geometric images are stored here where they can be used by someone in contemplating mathematical ideas. For instance, when thinking of a circle, the circle that comes to mind is one from the realm of Understanding. It is not the actual Form because the actual Form of circle in unchangeable and unalterable. But this mental copy can be altered as the thinker may wish. It can be modified, destroyed; all in the realm of the imagination. Above the Understanding we have the Dialectic. This is where the actual Form exists. This is where we would find the actual perfect circle. This is the only source of real knowledge.
Thus knowledge is progressive. Someone may give me a picture of a drawn circle. This drawn circle is Conjecture. They may then give me a piece of paper with the circle drawn on it. This is Belief. I can then imagine a line intersecting a circle. This circle would be in the realm of Understanding. However, through all of these progressions my knowledge about the circle would increase. But I can never have true knowledge of the circle from Conjecture, Belief, or even Understanding. True knowledge of the circle is only found in the Dialectic which is the Form itself.
How all of theses levels on knowledge interact is demonstrated in Plato's famous allegory of the cave. In this illustration, we are to imagine an underground cave. In this cave are prisoners who have been chained from birth in a manner that they cannot see each other or anything else behind them. They can only look forward to the wall at the back of the cave. Behind the prisoners, and elevated toward the mouth of the cave, is a fire. Unknown to the prisoners, between the fire and the prisoners is a wall and just behind the wall are people in a trench walking back and forth carrying "all sorts of artifacts, which project above the wall, and statues of men and other animals wrought from stone, wood, and every kind of material."(38) Theses image are projected on the wall of the cave so that only these images are visible to the prisoners. So then, the prisoners, unaware of the wall and fire behind them, believe that the images on the cave are real objects. They have no way of knowing otherwise. If someone where to escape and make his way to the mouth of the cave, he would realize that the rest of the people where only looking at images cast upon a cave wall by a fire. He would have a higher degree of understanding then the masses in shackles. If this escaped prisoner continued his journey until he came out of the cave, he would get an even better understanding of his surroundings. Plato writes "At first he'd most easily make out the shadows; and after that the phantoms of the human beings and the other things in water; and, later, the things themselves. And from there he could turn to beholding the things in heaven and heaven itself, more easily at night-looking at the light of the stars and the moon-than by day-looking at the sun and sunlight."(39)
We can interpret this allegory in respect to the dividing line. The prisoners who are in the cave in shackles represent the mass of humanity. They are perceiving things in Conjecture. When the prisoner escaped to the mouth of the cave and saw that the prisoners were only seeing images of objects, he had reached the stage of Belief. The fire here represents our sun and the artifacts represent the objects illuminated by the sun, while the images on the wall represent our perception of these artifacts. Up to this point, the prisoner has been confined to the world of the visible. It is not until he arrives outside of the cave that he starts to attain true knowledge. When he starts to see the reflection of things in the water he has attained some knowledge. When he actually sees the actual things themselves such as the sun, he has arrived at the Dialectic. He has attained true knowledge of the things itself from its Form.
According to St. Thomas Aquinas, all of our knowledge originates in sense perception.(40) Aquinas writes "Now it is natural to man to attain to intellectual truths through sensible objects, because all of our knowledge originates from the sense."(41) Furthermore, we derive our knowledge from the actual things themselves. The human mind has no innate ideas. We are all born with a clear slate. However, we possess a natural ability to abstract ideas.(42) We first know the thing, as matter, then abstract from the object its Form. In the words of D'arcy "We know the original, and it is in a reflective act that we are aware of the medium which is the form."(43) Tom Howe writes of Aquinas' view, "the form impressed upon the senses is not generated by the senses, or by the mind of the knower, but by the object in reality."(44) When we see an object such as a rock, the actual rock is what our senses observe and perceives its reflection. The mind knows that what it is seeing corresponds to reality and as a result, we come to knowledge about the rock. How do we know that the mind properly observes the rock? D'arcy writes "The intellect knows itself and knows its nature, which is not to create but to apprehend reality truly. Our particular certainties are guaranteed not by any general criterion, but by the object evidence in each case."(45) Pegis clearly articulates this view: "In knowing sensible being, how do we know it as sensible, which it is, and as being, which it likewise is? St. Thomas' answer is that sensible being exist in our knowledge as sensible being; its actuality exists in our sensible knowledge as sensible actuality, and not as an abstraction. And it is because we can give sensible actuality, within our knowledge, to what is sensible actuality in the order of existence that we can say we know sensible beings."(46)
The process by which knowledge is attained is first through the senses. The form of the real object is impressed upon the senses as a signet ring upon wax. It is important to note that this form is not generated by the senses, or the mind of the perceiver, but by the object in reality. All external sense knowledge of the object is combined by the Common Sense. The Common Sense is the unifying process of the senses into a single perception. That is, information gathered by all of the senses is combined into something of a single perception of the object. This collective perception of the object is then presented to the mind. This representation is then presented to the intellect, which abstracts the universal from the form as an idea. This idea comes to exist in the mind as a word. The word is the form of the universal object that has been perceived.(47) Thus, we go from the particular, such as Peter, to the universal, such as man. We go to the universal because universals are how we come to understand concepts. Our knowledge is gained from universals, not particulars. Copleston writes "As he held that the intellect knows directly the essence, the universal, St. Thomas drew the logical conclusion that the human mind does not know directly singular material things."(48) St. Thomas held that the mind has only an indirect knowledge of particulars, with the direct object of knowledge being the universal: "Our intellect cannot know the singular in material things directly and primarily. The reason of this is that the principle of singularity in material things is individual matter, where as our intellect . . . understands by abstracting the intelligible species from such matter. Now what is abstracted from the individual matter is the universal. Hence, our intellect knows directly the universal only. But indirectly, and as it were by a kind of reflection, it can know the singular."(49) This simply makes the point that we abstract from particulars to universals. We may see a particular dog, but our knowledge of this particular dog is based on our universal understanding of dogness.
The main distinction between the epistemology of Plato and Aquinas is in what and how we come to knowledge. According to Plato, we can only come to knowledge through reason. Our senses are deceitful and unreliable. We can only gain knowledge of the unchanging, perfect, and immaterial Forms. Aquinas, on the other hand, dismissed the notion of the existence of immaterial universals. He insisted that true knowledge comes from the objects of reality themselves. Our senses can be relied upon. We use our senses to view actual reality and recreate that reality in our mind, thus what is in our mind corresponds to what is in reality. For Plato, we start with the universal and abstract the particular. For Aquinas, we start with the particular and abstract the universal. For instance, according to Plato, when I see my dog spot, I know it is a dog because I am aware of the universal concept of dogness and that spot participates in the Form of dogness. So I am able to go from the universal dogness to the particular spot. But for Aquinas, I start with spot. I recognize spot as spot and I am able to abstract from spot the form dogness. Thus I know that spot is a dog. But I start with spot, not with dog.
Both Plato and Aquinas used the concept of universals in their epistemology. They were however used differently. When Plato talked about the universals, he is talking about a universal Form that exists as a real immaterial, immutable object. Aquinas used the concept to refer to the class of an object. For instance, when talking about John, Plato would insist that we can know nothing about John, we can only know humanness. John is changing therefore unknowable. According to Aquinas however, John is knowable. We see the particular, John, and from that we abstract the universal form humanness. But this universal is a class of objects with potentiality and actuality. It has essence and existence.
Another area where the two philosophers disagree is on the concept of innate knowledge. Plato contended that we have knowledge that exists before our birth. Our soul was directly aware of these universal Forms but this perfect knowledge was corrupted by birth. Aquinas rejected this notion. For Aquinas, all humans are born with a clear slate but with a capacity to abstract knowledge from the particular objects of matter around us.
Aquinas also made the point that if knowledge of truth was the end and purpose of the soul, as Plato insisted, how could this be attained in a spiritual substance? If it is our purpose to achieve knowledge, and our bodies serve as a road block to this knowledge, why do we have bodies? According to Plato, these bodies make true knowledge difficult. The body itself posses an obstacle to knowledge since it is the source of our senses, which are not the source of knowledge. In the words of Pegis "It is not, therefore, in the line of the essential purpose of the soul as an intellectual substance that it soul be joined to a body?"(50) In the view of Aquinas, the only explanation for the union of the soul and body is that through this union we have a completely intellectual substance. According to Plato, the knower is pure reason. According to Aquinas, the knower is the composite of soul and body. Man must be partly material in order to know. This explains why he must depend upon his bodily senses in order to come to knowledge. According to Plato, these senses are to the detriment of knowledge; according to Aquinas, they are essential to knowledge. According to Plato, man is only a thinker. He is a man who thinks abstract thoughts in separation from existence. In contrast, according to Aquinas, man is himself the knower.
In conclusion, both Plato and St. Thomas Aquinas made considerable contributions to the areas of metaphysics and epistemology. Plato, in an effort to address the import issues of how we know, proposed the idea of Forms. This he proposed in order to explain certain concepts and laws which are eternal and unchanging. Aquinas responded with his own metaphysics which made a distinction between essence and existence, act and potential, form and matter. This not only answered the questions that Plato attempted to answer, but it also answered questions that Plato's metaphysics could not, for instance, the concept of change. Everything has in its existence the potential for change.
Plato introduced his concept of knowledge to explain his metaphysics. He maintained that we cannot know true knowledge from objects of matter. They are changing and thus unreliable as a source of understanding. Knowledge can only be known of the eternal Form which exists separately from the object itself. St. Thomas Aquinas rejected this epistemology. He clearly demonstrated that knowledge is attained through the senses. If this were not the case, the body would be useless. The epistemology of Plato leads us into endless speculation, abstraction, and lack of knowledge; while the epistemology of St. Thomas Aquinas leads us into true knowledge of true objects, based on actual existence.