Essentials of Philosophy and Christian Thought - Lesson 5

Aristotle, Plotinus, and Aquinas

The contributions of Aristotle (the law of non-contradiction; the difference between essential and non-essential properties), Plotinus, and Aquinas

Ronald Nash
Essentials of Philosophy and Christian Thought
Lesson 5
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Aristotle, Plotinus, and Aquinas

Aristotle / Plotinus / Aquinas


I. Aristotle

A. Introductory

B. Two major contributions

1. The law of non-contradiction

2. Difference between essential and non-essential properties

C. The square of opposition


II. Plotinus

A. Averroes

B. Siger de Brabant


III. Aquinas

  • Dr. Nash covers the objectives to the lectures, reading material, and discusses the five major beliefs that make up a worldview

  • A discussion of naturalism, what it teaches, two major exponents of naturalism (Lamont, Russell), and why it is wrong

  • Theories opposed by Plato ("Hermman"), his significance, his three forms of dualism, and Plato's basic teaching on "forms"

  • Plato's doctrine of the immortality of the soul, his understanding of knowledge (argued against by Augustine and Plotinus), and a discussion of rationalism vs. empiricism

  • The contributions of Aristotle (the law of non-contradiction; the difference between essential and non-essential properties), Plotinus, and Aquinas

  • Open theism is the belief that God does not know the future; otherwise, we would not be free in our decisions. Nash discusses the teachings of God's omniscience and human free will, and the logical implications of this teaching.

A seminary-level version of this class is available in our Institute Program.

Dr. Ronald Nash

Essentials of Philosophy and Christian Thought


Aristotle, Plotinus, and Aquinas

Lesson Transcript


Now, we jumped directly from Plato to Augustine. Plato died in 347 BC; Augustine died in 430 AD. That’s a big jump chronologically, but it’s not such a big jump as far as ideas go because Augustine was clearly a Christian Platonist. The adjective ‘Christian’ is very important. One can be a full-blown Platonist and not be a Christian even though Plato’s worldview is certainly sympathetic with a lot of Christian concerns. But that jump does make sense.

Now what we’re going to do is say just a very little bit about Aristotle—not a whole lot, for two reasons: Aristotle’s thought is really too complicated to try and summarize in too much detail in this very short summary course, but I have to say at least a little bit about Aristotle before we jump to Thomas Aquinas who died in 1274 AD. If Augustine is a Christian Platonist, properly modified, then Thomas Aquinas is certainly going to be representative of a kind of Christian Aristotelianism, even though it is important to remember that there are a whole lot more influences on Aquinas than just Aristotle.

Aristotle died in 322 BC. He died one year after the death of Alexander the Great—who, people should understand, was a pupil of Aristotle. Aristotle was technically not a Greek; he was a Macedonian. His father was court physician to the king of Macedonia, and when Aristotle reached the age of 17, his father sent him to Athens where he entered Plato’s university called the Academy and became, I’m sure, Plato’s greatest pupil. Plato and Aristotle didn’t agree on a whole lot. If Plato was the consummate rationalist, which still means that Plato’s rationalism needed some serious modification, Aristotle was the consummate empiricist. Aristotle thought that he would most certainly become Plato’s successor at the Academy, Plato’s university, when Plato died, but, when Plato died, the control, the leadership of the Academy passed to a nephew, and Aristotle left Athens in a rather unhappy frame of mind. After Plato’s death Aristotle wandered around Asia Minor for a while, married the daughter of a king in Asia Minor, returned to Macedonia for a few years where he tutored the young son Alexander, who did become Alexander the Great, and then returned to Athens where Aristotle founded his own university which is called the Lyceum.

Now if I had more time I would talk to you about two major contributions that Aristotle made to philosophy. Aristotle, first of all, discovered the law of non-contradiction. And I have a lot to say about the law of non-contradiction in many of the books that I have written. It is the fundamental law of all human thinking. Anybody who denies the law of non-contradiction cannot really speak meaningfully, cannot think intelligently.

One of the things Aristotle noticed about the law of non-contradiction is it is not just a law of thinking; it is also a law of being. Many people failed to recognize an important dimension to what Aristotle meant by the law of non-contradiction. Aristotle understood that the difference between B and non-B is more than just a minor differentiation or a minor distinction. When you deny the law of non-contradiction, when you say that A can be both B and non-B at the same time and in the same sense, you’re really embarking on a serious journey on the road to madness. Perhaps the best way to explain what Aristotle meant by the difference between B and non-B is this: Say that B is any given class of objects that you want it to be. It could be the class of all horses; it could be the set of all trees; it could be the class of all human beings. A class is a collection of things that possess the same essential property. Now, when a person denies the law of non-contradiction, Aristotle said, you do more than just say that a human being can be both a man and a non-man. Aristotle pointed out that what you mean by a non-man is in fact everything else in the universe that is not a human being. This is a recipe for nonsense. If somebody says Socrates is a man, and then, because you deny the law of non-contradiction, you also say Socrates is a non-man, you’re uttering nonsense. Here’s why. The class of non-man turns out to be everything else in the universe that is not a man. So when you deny the law of non-contradiction and say Socrates is a man but he’s also a non-man at the same time and in the same sense, what you’re saying is Socrates is a man, yes, but he’s also a horse, he’s a dog, he’s a collection of viruses, he’s the universe, he is everything else in the universe that is not a man. That is to deny every basic distinction in the universe—every basic distinction.

Let me relate this to God. If somebody says Jehovah is God and non-God at the same time and in the same sense, what your statement really amounts to is the claim that there’s no difference between God and the devil, there’s no difference between heaven and hell, there’s no difference between being saved and not being saved. Such is the nonsense that follows when people deny the law of non-contradiction.

Now, it’s especially important to notice this today because there are people within the Christian community who think it’s smart or cute or intelligent to deny the reality, to deny the indispensability of the law of non-contradiction. But when you do that, whether you do it in the name of postmodernism—something about which I plan to say a few things later on—or whether you do it in the name of something else, you’re actually saying that distinctions make no difference at all. But as Aristotle correctly recognized, if you deny the law of non-contradiction, you deny the possibility of significant human behavior, because then there’s no difference between traveling east or traveling west, you deny the possibility of intelligent speech, because every word that you might then use ends up having an infinite number of meanings which means you cannot communicate through speech. So Aristotle needs to be credited with a great deal of gratitude for his discovery of the law of non-contradiction. And let me tell you this, listeners: Anybody—and I don’t care whether you regard that person as trustworthy Christian or not—anybody who denies the indispensability, the necessity of the law of non-contradiction is doing a great disservice to the Christian faith and is holding the position that I believe ends up resulting in heresy. The law of non-contradiction. Well, read the books. Take a listen at the major tape.

Now Aristotle’s other great contribution—and he made many—but his other great contribution was recognizing the important difference between essential and non-essential properties. Let me give you a synonym for a non-essential property, we could just say a contingent property. Take me for example. I happen to be bald. I once had hair, but I don’t have hair anymore. But baldness is not an essential property of being a human being. Whatever an essential property is, it is the defining property that places any object into a specific category. Now, the essential property of being a human being is humanness. Baldness happens to be a non-essential property which boils down to saying that I can have hair or I can not have hair but I’m still essentially a human being.

Now one of the ways in which Aristotle’s distinction between essential and non-essential properties, one of the reasons that’s important relates to a very fundamental question about Jesus Christ. I’ve had many, many people over the years come up to me and ask me to explain why there is not a contradiction in the Christian belief in the incarnation, because these people say to me: You believe that Jesus Christ if fully God—to which I respond yes I do—and then they add, you also believe Jesus Christ is fully man, fully human—to which I respond yes I do—but if you say that Jesus is fully God and fully man, are you not contradicting yourself, and if you are contradicting yourself about this very essential doctrine of the Christian faith, does that not falsify the Christian faith—to which my reply is no, it does not.

Now, there is a way we can use Aristotle’s distinction between essential and non-essential properties to show there is no logical inconsistency in the Christian belief in the incarnation, that we can believe Jesus is fully God and fully man and not contradict ourselves. But I’ve got bad news—I only have three hours for this particular tape, and it takes about thirty minutes using some distinctions from Aristotle to explain why there is no contradiction in the Christian faith. So let me tell you how you can get that information. You can get it first of all from the longer tape, to the forty hour course called The History of Philosophy and Christian Thought. You can get it there, or you can get it from my textbook Life’s Ultimate Questions, or you can get it form another textbook called Is Jesus the Only Savior?, or you can get it from still a third book called Worldviews in Conflict. It must appear obvious to you that I think this information is very important, but there just is not time to do it at this particular time, so I just commend you to those books or that longer tape and hope you get that information as soon as you get exercised over this question of whether or not Jesus Christ is fully God and fully man. And be thankful that we have this helpful information from the Greek philosopher Aristotle that can help us explain why there is no contradiction here.

I’m now going to say a little bit about Plotinus. Very few people know the name Plotinus, and that’s understandable because he’s a very difficult thinker to understand. I’ve had people tell me that Plotinus’ Greek is the most beautiful Greek every written by an intellectual, someone who’s not a poet perhaps. Plotinus was born in upper Egypt in 2005 AD [sic], okay; he died in Rome in 270 AD. His basic position is one we call panentheism. Panentheism is different from pantheism. Pantheism is basically the belief that God and the world are identical, God is the world, the world is God. Panentheism is a little more sophisticated—it’s wrong, I’m not commending it to you at all. One way to explain panentheism, the position of Plotinus, is this. He believed that God is the mind of the world, and the world is the body of God. What that boils down to is the belief that God and the world are co-eternal and co-dependent. God needs the world, which is his body; the world needs God, which is the mind of God.

Now, I’m leading up to the influence that Plotinus had during the middle ages. I’m going to do that without going into a great deal of detail that happens to be in the longer History of Philosophy tape. During the late middle ages a large number of thinkers in Islam came under the influence of Plotinus’ ideas. This was due to great extent because there was enormous confusion during the middle ages as to which earlier philosopher had written certain documents. For example, students of philosophy during the middle ages had discovered a writing that they thought had been composed by Aristotle that came to be called The Theology of Aristotle, but it was actually written by Plotinus. In ways like this there was enormous confusion in the Islamic world where there had been a great revival of learning. There was a great deal of confusion as to which documents from ancient Greece had been written by Plato, and which had been written by Aristotle, and which indeed had been written by Plotinus.

This confusion over Plotinus, Aristotle, and Plato, occurred during the lifetime of a Muslim philosopher named Averroes who lived during the 12th century, the 1100s. Averroes turned out to be something of a Muslim heretic actually. He held to three major beliefs. First of all, Averroes correctly understood that Aristotle’s view of creation, of the origin of the world, was quite contrary to both Islam and Christianity. And so Averroes, thinking he was following Aristotle’s lead, taught that the world had no beginning, the world was eternal, the world had always existed. This got Averroes in trouble with Islamic leaders. Secondly, Averroes not only denied the doctrine of creation, Averroes denied the doctrine of the immortality of the soul. And thirdly, Averroes taught a doctrine called the doctrine of the double theory of truth. You see, when certain Muslim theologians came to recognize that Averroes’ teaching was heretical as far as Islam was concerned, they posed a possible threat to his life. So, Averroes tried to wiggle his way out of this difficulty by saying something can be true in religion and false in philosophy, or something can be true in philosophy and false in religion. So that when the Islamic leaders came to Averroes posing some threat to his life, he said no, you don’t understand, I believe that Allah created the world—but that’s religion—in philosophy, I deny that Allah created the world, in philosophy I teach that the world is eternal. And he confused the mullahs, he confused the leaders of Islam, and, you know, he lived a long, rich, full, happy life, I guess. Likewise, when they came to him and accused Averroes of heresy with respect to the doctrine of life after death, he said: Oh no, I’m not in conflict with Islam—I believe that religiously the doctrine of life after death is true, but philosophically I do not believe that life after death—. Now, that word, for some reason, now some people question whether Averroes actually taught that, but I—and some people, say, well he was too smart to do that—but I’m sorry, I think think the literature is clear that Averroes did play that game, or maybe he really was, you know, just confused on that.

Now, what is relevant to all of that is that these heretical ideas of Averroes, these doctrines of Islam that were denied by Averroes, this heresy was picked up by some professors at the University of Paris. And the major troublemaker at the University of Paris was a man named Siger of Brabant. Siger of Brabant is called, in the literature, a Latin Averroist. What that means is he was a heretic within the Christian University of Paris who was teaching the heretical ideas of the Muslim thinker Averroes.

Now all of this is related to Thomas Aquinas who was born in 1225 and who died in 1274. Thomas Aquinas, who eventually studied at the University of Paris and then taught at the University of Paris, recognized immediately the serious theological threat that Latin Averroism, the heretical ideas of Averroes now taught within a Christian context, Aquinas recognized the threat that his posed to the integrity of the Christian faith in the 13th century. Aquinas began to do battle with these heretical ideas of Averroes as taught at the University of Paris by this bad dude named Siger of Brabant. But you’ve got to understand that Aquinas had a problem here because, by this time in his life, Aquinas had become persuaded that the philosophy, the philosophical system of Aristotle was the best philosophical framework for Christians to build a Christian worldview of. And so Aquinas had to do battle with these ideas but he had to do that battle in a way that would not destroy the influence or the usefulness of Aristotle’s work within the Christian world. You see what I mean? How can I still recommend Aristotelianism to Christian philosophers and yet condemn the denial of creation and condemn the denial of human immortality and deny the horrible doctrine of different kinds of truth being true in philosopher and religion?

So here is how Aquinas went about his work. First of all, to his great credit, to his great credit, Aquinas denied the double theory of truth. Let there be an end to this nonsense of people saying that something can be true in philosophy and false in religion, or vice versa. According to Aquinas, if something is true in philosophy or science, it must also be true in the Christian faith. You cannot have one thing being true in one area and it being false in another area. Truth is truth—all truth is God’s truth. So Aquinas did battle with that. If something is true in one area then it must be true in the other.

Secondly, how did Aquinas do battle with the Aristotelian denial of immortality, or, at least, a Plotinian—Plotinus’ denial of immortality? Well, Aquinas had found a little passage in Aristotle’s writing on psychology, and this is how it’s translated. Aquinas found a small text in the third book of Aristotle’s work on psychology. It referred to something called the active intellect. This is the quotation: “The active intellect is separable and immortal and, without it, nothing thinks.” That’s an exact quote, and I’m doing that from memory. The active intellect is separable and immortal and, without it, nothing thinks. Now what’s going on here? Well I give you the full details, of course, in the longer tape, or in the various books. By active intellect Aristotle meant something different from the soul. Plato had talked about the immortality of the soul. Augustine had talked about the immortality of the soul. But Aristotle took a totally different look at the soul. For Aristotle, the soul meant basically life. And Aristotle thought that there were different kinds of souls in the created universe, there were different kinds of life. He talked about plants having a life, a soul. He talked about animals having a soul. And he talked about human beings having a soul. Now, when Aristotle talked that way, and when Aquinas talked that way, he didn’t mean that dogs and tomato plants have a soul in the same way that human beings have—these are different structures of life that one finds in the universe. Plant life simply means that plants live in a certain kind of way—that’s all he meant. He didn’t mean that plants have an immortal soul. And dogs have a certain and more complex kind of life than plants have. And human beings have an even more complex kind of life. And if you want to make sense out of this you’re going to have to go to the tapes or to the books—I just can’t take forty or fifty minutes and go into greater detail on this, okay.

Now, besides there being a soul for plants and animals, Aquinas focused his attention not upon the soul but upon the active intellect—it’s a part of the mind, it’s a part of intelligence. Now let me quote again what Aristotle said: “The soul is separable”—what does that mean? Among the most sensible possibilities of what Aristotle meant here is the fact that the active intellect is separable from the body. Frankly, I don’t know what else Aristotle might have meant there. The soul is separable. The active intellect is separable probably means separable from the body. And the active intellect is immortal and, without it, nothing thinks.

Well, it’s hard to imagine a more ambiguous passage in the whole writing of philosophy, and, in fact, as I go on to explain in my writings and in my tapes, disputes over the interpretation of this passage actually gave rise to the three major systems of philosophy during the next ten centuries or more. Plotinus interpreted the active intellect to be, actually, the presence of some kind of cosmic mind in the universe. And St. Augustine interpreted this passage, the active intellect, to be God active in the human mind. And the Aquinas interpreted the active intellect to be a separable and independent part of the human mind. Now I know that this is all confusing, but when you read the book and read the tape it’s all, you’re going to become an expert on Aristotle’s active intellect. The bottom line here is that whereas Aquinas rejected the double theory of truth and said truth is truth and it’s got to be the same in science, in philosophy, and religion, he found this strange passage in Aristotle’s book on psychology and he interpreted it to mean that there is something immortal within every human being and it is the active intellect. So, therefore, therefore people who follow Aristotle do not have to deny nor do they have to think that Aristotle denied personal immortality. Now I go on and explain in other places that this is a highly disputed interpretation of Aristotle, that Aquinas is probably wrong in his interpretation of Aristotle, but such is life.

Now, finally, what did Aquinas do when confronted by the fact that Aristotle did say the world, the cosmos, the universe was eternal? Aquinas could not deny that. He found a way to deny that Aristotle had denied human immortality, but it was too obvious and too clear and too indisputable that Aristotle had really denied the creation of the world. But Aquinas was a genius. He found a way to extricate Aristotle from this dilemma, and here is how he did it. Here’s how he did it. He said there’s a distinction between faith and reason. Reason is the domain of philosophy and faith is the domain of theology. Where do we get our information about philosophy? We get it through sense experience. Where do we get our information about the content of faith? We get it from the Bible, among other places; it’s basically a part of our theology. Now, what Aquinas did to extricate Aristotle from this embarrassing situation in which the philosopher who was the father of what Aquinas wanted to be, the major Christian approach to philosophy, his real mistake was this: The doctrine of creation is something we can only know through faith. It is something we can only know through theology. It is something we can only know through, or from special revelation, from Scripture. Aristotle did not know anything about special revelation. He did not know anything about the Old Testament. Aristotle didn’t know anything about the New Testament. So Aristotle made a mistake, but it turns out not to have been that serious a mistake. Aristotle thought philosophers could prove that the world did not have a beginning in time. Aristotle thought that philosophers had proven that the world was eternal. Aquinas said you can’t do that, that’s theology. And Aristotle didn’t know any theology so he didn’t know what he was talking about, so he was wrong. It was a mistake, but it wasn’t that serious a mistake, because he walked in territory where no human being can know the answer to this questions without having access to special revelation, to the Bible. So Aristotle was wrong, but we can forgive him because he tried to answer a question which no human being can answer on the basis of philosophy and science. Okay?

So what this amounts to is the great Aquinas pulled the rug out from underneath those heretics at the University of Paris, those heretics within the medieval church. He also rebutted forever the bad mistakes by Averroes. The Muslims should have appreciated but—they should have done what Aquinas had done. So, the Averroist heresy which had invaded the University of Paris had the rug pulled out from underneath it. Aquinas successfully refuted the heretical errors of Siger of Brabant. He turned out not to survive very long because a rather insane clerk assassinated him, that is, assassinated Siger of Brabant. So that’s the story of how Aquinas got started in philosophy.

A lot of Aquinas’ system turns out to be Aristotelianism reheated—not all of it. I don’t mean to minimize Aquinas’ contribution. Often when I read contemporary philosophical works and I find contemporary philosophers making interesting and important distinctions, and then I discover that Aquinas himself had made these distinctions in the 13th century, I just marvel at the man’s genius. I offer a lot of observations about a lot of the details of Aquinas’ philosophy in my book Life’s Ultimate Questions and in the longer tape. But time makes it impossible to go into any greater detail on Aquinas.