Lesson 1: What is Logic?


These are supplementary remarks for the course produced for the International Catholic University on the logic of Aristotle and St. Thomas. The course will have a total of twelve lessons. The first lesson is on the importance of a course in logic for the study of Thomistic philosophy and theology. It will give you the big picture, an overview of this course and of the whole of logic. But before I begin that task, I want to give a brief historical survey of the origin of Aristotelian logic.

We first find logical themes being discussed by the ancient philosophers Socrates and Plato. Socrates discusses definition and induction in particular, while Plato take up several logical topics in detailed fashion, most notably in the Meno and the Republic. But neither Socrates nor Plato has given us a complete treatise on logic. It took Plato's student Aristotle to systematize the study of logic. Aristotle wrote several treatises on the different parts of logic, and these are collected in what scholars call the Organon (a Greek word meaning tool). Aristotle's Organon was the basis of most logical studies for over 1500 years, and it has been the subject of numerous commentaries. St. Thomas' teacher, St. Albert the Great, writes commentaries on every single book of the Organon. Perhaps this is why St. Thomas himself, though clearly familiar with the entire Organon, wrote commentaries on only two treatises in it, the Peri Hermeneias, translated as On Interpretation, and the Posterior Analytics. What we are going to do in this lesson is to discuss the prologue St. Thomas wrote to his commentary on Aristotle's Posterior Analytics. We will discuss that now because it is not a prologue just for the Posterior Analytics, but for the whole of logic. Also, St. Thomas covers the two themes we intend to discuss in this class, the importance of the study of logic, and the outline of the whole of logic.

Instinct, Reason, and Art.

St. Thomas begins his prologue by talking about the importance of logic, and using a quotation from the Metaphysics of Aristotle. Aristotle writes: The human race lives by art and reason. To explain this quotation, St. Thomas draws a contrast between man and the other animals. Man lives by art and reason, while the other animals live by instinct. This is how St. Thomas puts it:

While the other animals are driven to their actions by a certain natural instinct, man is directed to his actions by the judgement of reason.

Let me give an example. A sheep sees a wolf: it does not reason to itself that the wolf is dangerous, and I should run away from dangerous things, therefore I should run away from the wolf. The sheep simply has an overwhelming feeling of fear which drives him to run away by instinct from the object of that fear. A man also has feelings of fear, but the difference is that man can overcome his fear by the direction of reason. A man might see the wolf, feel fear, but judge that at this time the wolf is not dangerous, or that it will be more dangerous to run than to face the wolf. Man ultimately directs his actions by reason, while the animals are driven to their actions by instinct.

At first, this seems to point to the superiority of man over the other animals. The problem is that bare reasoning is not as quick as instinct and is more prone to error. That is, a man asks himself whether he should run from the wolf, and by the time he is through deliberating, the wolf has eaten him. Or perhaps he has enough time, but his reasoning is mistaken. Reason, then, requires deliberation, takes time, and is more prone to mistakes than instinct. Thus, it might now seem that because man is directed in his actions by reason, he is inferior to the other animals.

What is the answer to this problem? St. Thomas puts it this way:

The various arts serve to perfect human actions so that they proceed easily, and in an orderly way.

To understand that sentence, we need to understand what St. Thomas means by art. He is not referring just to the fine arts, painting, sculpture, music, but to a general habit of being able to make things or to perform certain kinds of actions. For example, the ability the carpenter has to make a table or a chair is an art according to St. Thomas, and the ability that the builder has to build a house is another kind of art. St. Thomas is saying that art gives reason the kind of ability that the animals have through instinct. Art enables reason to direct human actions so that they proceed quickly, easily, and without errors. St. Thomas then defines art as follows:

Art is nothing but a sure and rational ordering of the way human actions arrive at their correct ends through determinate means.

St. Thomas uses the art of building as an example. Every man needs shelter. If we look at man before he had discovered the art of building, we can see him making a very simple and imperfect shelter, for example, a lean-to. He gathers materials he thinks might help, big leaves, vines, a few sticks, and ties them together. He lashes this to a large tree. When he uses the lean-to, he finds that its better than no shelter at all, but that the roof leaks, it lets in drafts, and it tends to fall down. Here is the lesson: using reasoning alone, a man cannot make an adequate shelter. On the other hand, bees by instinct immediately make a very efficient shelter, their hive. Before an art is discovered, reason is in some sense inferior to instinct.

Because man has reason, however, he can reflect on how he built the lean-to. He can think about which materials and construction methods worked, and which did not. At the end of the process of reflection the man has begun to acquire the art of building. Next time he builds, he will use only those materials and those methods which worked the first time. He will build a better lean-to, and he will build it more easily. As he progresses further in his actions of building, the art of building becomes more and more perfect and the shelters are more and more effective.

We can sum it up this way: because man has reason rather than instinct, initially his actions are very imperfect. But because he can use reason to reflect upon his actions, he can perfect the means he uses to accomplish his purposes, and that reflection results in the discovery and acquisition of the arts.

The Necessity of Logic

What we can see so far is that art in general is necessary, that men live by reason and by art. We now need to see why a particular art, the art of logic, is necessary. St. Thomas writes:

Reason can not only direct the lower powers of the soul, but reason can direct itself in its own action, because it belongs to the understanding part to reflect upon itself.

Two important points are made in that sentence, so let us take them one at a time.

First, reasoning directs its own actions. St. Thomas is pointing out that reasoning is not just something that directs other actions, reasoning is itself an action. Thinking and reflecting are also kinds of doing.

Second, unlike the powers of soul which we share with the other animals, human reason can reflect upon itself. A sheep sees a wolf, feels fear, and runs away. His power of sight sees the wolf, but it does not see the action of seeing. His appetites fear the wolf, but they don't fear the action of fearing. He runs, but running does not perceive the action of running. The actions of the sheep cannot reflect upon themselves. But reason, because it is a spiritual power, not only can perceive the other actions, it can reflect upon itself and perceive its own action. Reason can reason about the action of reasoning.

The consequence is that, just as reason can reflect upon the action of building to discover the art of building, so reason can reflect upon the action of reasoning to discover an art of reasoning. That art is logic.

Let us remember what an art does for us. Before man possessed the art of building, he could build, but only with great difficulty, in a haphazard way, and he produced an inferior product. After he discovered the art, he could build easily, in an orderly way, and without mistakes. The same is true about reasoning. Before man discovered logic, he could reason, but with great difficulty and many mistakes. After he discovers logic, the art of reasoning, he can reason with much greater ease and make many fewer mistakes. St. Thomas writes:

There must be some art which directs the actions of reason itself. Through this man can perform the actions of reason in an orderly way, with ease, and without error. This art is logic.

Now we have seen fairly clearly why logic is necessary. If we are going to study St. Thomas' philosophy and theology, we will have to reason about very difficult matters. Reasoning by itself is not easy, and so it needs an art so that it becomes easier. Moreover, reasoning is prone to error, and so it needs an art which will prevent these errors. That art is logic. If we want to proceed well in philosophy and theology, we absolutely need that art of logic to guide us.

The Parts of Logic

After St. Thomas explains why we need the art of logic, he goes on to discuss the parts of that art. In fact, two thirds of the prologue is concerned with the outline of logic, showing us the parts of logic and the order of those parts. Our next task, then is to go over the outline St. Thomas provides for us.

Logic is going to be divided into many parts. What principle divides logic into parts? St. Thomas says that, since logic is the art of reasoning, logic will map onto the various ways in which reason works. That is, there will be one part of logic for each different kind of action the human mind performs. So the first thing that St. Thomas does is outline what he calls the three operations of the human intellect.

The first operation of the human intellect is, as St. Thomas puts it, the understanding of what is simple and indivisible, which understanding conceives what a thing is. For example, if I am going to study the science of geometry, I need to know what a triangle is. The idea of a triangle, what a triangle is, is simple, simple in the sense that it does not combine a subject and a predicate. It is just a subject to be understood.

It might seem strange that we would need a part of logic devoted to understanding what is so simple, the bare idea of what something is, such as what a triangle is. But St. Thomas would respond that we begin with an indistinct understanding of what a triangle is, and that the idea of a triangle is perfected and made distinct when we find a definition of a triangle. In this case the definition is plane figure bounded by three straight lines. Now the definition is a complex phrase, but it does not combine a subject and predicate, it does not say A triangle is a plane figure bounded by three straight lines. It simply says plane figure bounded by three straight lines. Thus, this definition is still understood by the first operation of the intellect. We see, then, that even in the first operation reason makes a progress in knowledge, and thus it needs a part of logic which shows it how to make good definitions.

That part of logic is covered in two treatises, one written by Aristotle called the Categories, and the other written by Porphyry, a philosopher of the later ancient world, which is called the Isogoge, which is simple Greek for introduction. The Isogoge is meant to be an introduction to the Categories of Aristotle, although it is also an important treatise in its own right. The Isogoge, though not by Aristotle himself, is often included in editions of his Organon.

The first operation of the mind was characterized by a kind of simplicity. The second operation, by contrast, is characterized by complexity, the combination or separation of subject and predicate in a statement. The second part of logic deals with the combination or separation of the simpler things understood by means of the first part of logic.

The goal of the second operation is also in contrast to that of the first. The first operation is merely trying to understand what something is, while the second is trying to understand what is true or false about that simple thing. Thus, when I say triangle, or even plane figure bounded by three straight lines, there is no judgement of truth and falsity involved. But if I were to say A triangle is a plane figure bounded by three straight lines, then I am making a judgement about the true and false. Thus, through the operation which combines or divides a subject and predicate to make a complete statement the mind comes to understand, not just what something is, but what is true or false. The second part of logic directs this operation of composing or dividing and Aristotle's Peri Hermeneias, or On Interpretation, is about this part of logic.

Then St. Thomas moves on to the third and final operation of the human mind. He calls this operation discursive reasoning. Discursive comes from the Latin which means running through. The third operation is called discursive because in it the human mind runs through from one truth into another. It runs through one thing it knows and comes to a knowledge of what it previously did not know. All the rest of the books of the Organon are devoted to directing this action of reason.

St. Thomas explains why Aristotle needs several books to cover this part of logic by comparing the operation of discursive reasoning with the works of nature. He writes:

The actions of reason are like the actions of nature. Now we find three kinds of actions in nature. In certain things nature acts by necessity, that is, in such a way that she cannot fail. In other things, nature works more frequently than not, though sometimes she falls short of her proper action. There are two kinds of actions in the latter case, one which happens for the most part, the other in which nature falls short of what is fitting.

Let us take some examples to understand what St. Thomas means. The sun never fails to rise in the morning an set in the evening. There seems to be no natural way that this process could be obstructed. Therefore, we say that the rising and setting of the sun happens by necessity. When animals begin the process of generation, the end result is usually the production of a complete animal of the same kind. Nature here usually performs her proper action, does what she is supposed to do. On rare occasions, however, because there is some genetic deficiency, a failure at the beginning of the process, nature fall short of her intended action, and produces a deformed animal. These are examples of St. Thomas' three natural processes.

Just as in nature there are three kinds of actions, so there are three kinds of actions in discursive reasoning. Some reasonings reach their intended result by necessity. That is, they never fail to reach the truth. Other reasonings achieve the truth for the most part, just as nature usually produces a complete animal. Sometimes, there is failure in reasoning, so that it comes to a false conclusion because there has been some failure in its beginning, as when nature produces a deformed animal. That is why there are going to be three parts to the third part of logic.

In the seventh lesson we will discuss the subdivisions of this part of logic in detail. For the moment, let us simply note the names of the treatises in which Aristotle discusses these parts of logic. The first part of the third part, called the judging part, is covered in two books, the Prior Analytics and the Posterior Analytics. The second, called the discovering part, is covered in three books, Topics, Rhetoric, and Poetics. The final part is covered in one book, called Sophistical Refutations.


St. Thomas has done a great job in explaining to us why logic is important, and in giving us an overview of the whole of logic. In the following three lessons we are going to get into logic itself, the logic of the first operation. The next class will be a discussion of the most important points in Porphyry's Isogoge. Then in the two succeeding classes we are going to discuss Aristotle's Categories.


1. Write a 250 word essay which comments on the following statement: I do not need to study logic because I have a natural aptitude for metaphysics.


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