Lesson 10: Dialectic


In our last lesson we finished our discussion of the judging part of logic. Judgment always has certitude, and the certitude of judgment can come from either the form of reasoning alone, that is, the syllogism simply, or from the matter of reasoning, as in the demonstrative syllogism. But we also noted in our discussion of demonstration that before we can judge whether a theory is true or false, we need to discover it. Thus before we can use the judging part of logic, we need to use the discovering part. Aristotle calls the chief part of discovering logic dialectic and discusses it in a book called the Topics. This lesson will examine Aristotle's Topics.

The Uses of Dialectic

Since I want to make sure that our discussion of logic is useful in your intellectual life, the first thing we are going to examine in dialectic is not its nature or definition, but rather its utility. Aristotle explains it as follows:

We must say how many and for what purposes the treatise is useful. They are three: intellectual training, casual encounters, and the philosophical sciences.

Aristotle here enumerates three different uses for the dialectical art. Let us take up these uses one at a time.

Aristotle thinks that the first use of dialectic is for intellectual training. Even a superficial reading of Plato's dialogues reveals that what he calls dialectic is an art of intellectual debate. And it is clear that debate sharpens the mind. Even today teachers recommend that promising students becomes involved in their school's debate society. If debating sharpens the mind, and dialectic is a skill in debating, then the practice of dialectic clearly contributes to sharpening the mind.

The second use is for casual discussions. Suppose that one wishes to discuss a philosophical question with somebody who has not studied philosophy. He cannot present philosophical demonstrations to that person because the latter will just not be able to follow them. He will need some other kind of logical tool in order to have a discussion about philosophy with that person. Aristotle says that dialectic is that kind of tool.

Finally, philosophers use dialectic in two ways. First, when a philosopher wishes to come to some conclusion in the philosophical sciences, it is often helpful for him to look at both sides of the issue. For example, if he is trying to demonstrate that God exists, he may also want to look at the arguments that purport to show that God does not exist. Aristotle says that dialectic is the tool for that kind of procedure.

Philosophers also use dialectic to discuss the first principles of philosophy. We said before, at the end of our last lesson, that all demonstration goes back to first principles, first premisses which are not demonstrated, but which we learn in some way through sense experience. What is clear from our discussion is that the principles do not come directly from sense experience, but rather that we have to go through a process to draw them out of sense experience. Aristotle does not say what exactly that process is in the Posterior Analytics. But he points out in the Topics that that process is often a dialectical process. Dialectic allows us to discuss the principles of demonstrative syllogisms, and even reason about them. It enables us, not to prove them, but to discuss and reason about them, and this helps us to learn them.

What these three uses have in common indicates two important properties of dialectic. First, dialectic gives you the power to discuss anything. When we are training someone's intellect, we are not training it just for one particular task. We are training the intellect as a whole to be sharper in thinking about any subject. Our casual discussions can also be about any philosophical subject.

Aristotle also says that dialectic gives us an ability to discuss the conclusions or principles of any discipline. The logical tool that is demonstration, in contrast, does not give us such a power. While the syllogistic form can be used in any science, the principles which form the necessary matter for demonstrations are each restricted to one science. For example, the postulates of geometry cannot be the premisses of a demonstration in metaphysics. The use of the demonstrative syllogism presupposes a firm grasp of the principles of a science, and therefore that tool does not by itself give us a power to reason on any subject. Aristotle claims, however, that the possession in itself of the tools of dialectic give the mind the ability to reason dialectically about any subject, whether or not we have a firm grasp of the first principles of that subject.

For instance, even if we have read and understood the Posterior Analytics in its entirety and understood it thoroughly, we could not give a demonstration in metaphysics unless we also had a firm grasp of the first principles of metaphysics. We can, however, reason dialectically about metaphysical subjects as soon as we have acquired the dialectical art, and even before we have a firm grasp of metaphysical principles because dialectic is the power of reasoning probably about any subject. Dialectic is a power to discuss any subject at all, but, as we shall see, only to reason about it with probability, not certainty.

The second thing we can notice about all of these uses of dialectic is that they involve a combat between intellectual opponents. When one debates for the sake of training his mind, he is arguing with an opponent about the truth of something. When we having a casual conversation about a philosophical matter with a non-philosopher, it is because the non-philosopher takes up a position contrary to the philosophical one. And when the philosopher is looking at the pros and cons of a particular philosophical question, once again there are conflicting opinions.

Thus all of the uses of dialectic indicate two things about it. First, it is a power that deals with any intellectual matter at all. Secondly, it is a power of disputing about intellectual matters. And since dialectic is part of the discovering part of logic, it is clear that the purpose of dispute is to prepare the mind for a further, more perfect knowledge about the subject under discussion, if such a knowledge is possible.

The First Tool: The Dialectical Syllogism

Now that we've talked about the general nature of dialectic, we need to talk about the specific tools that dialectic uses to accomplish its purpose. There are two main tools of dialectical reasoning. The first is the dialectical syllogism; the second is induction.

The dialectical syllogism, like all syllogisms, has certainty from its form. If the premisses are true, then the conclusion has to be true. But the difference between the dialectical and the demonstrative syllogism is that the dialectical syllogism does not have certainty from its matter. We can state this more precisely by giving a definition of the dialectical syllogism that is parallel to that of demonstration. Demonstration was a syllogism that produced scientific knowledge. Dialectic will be defined as a syllogism which produces opinion. And just as we had to define scientific knowledge, now we will have to define opinion.

First I want to contrast opinion with science. We already saw that science is reasoned-out and certain knowledge. When my intellect possesses certain knowledge about something, it makes a complete assent to that truth, an assent so complete that there is no fear of error. When I really understand the demonstration for the angles of a triangle adding up to 180 degrees, my intellect by its own power assents to it completely, without any fear that the conclusion might turn out to be false.

Clearly this is not the case with opinion. With opinion, I have a fear that the opposite of what I hold might be true. And that is evident because I often change my opinion about matters. Thus opinion lacks that complete assent which is the hallmark of science and belief.

But opinion also differs from doubt. My mind is in a state of doubt when it makes no assent at all to either side of an issue. When I am in a state of opinion, my mind does assent to one side. Thus we might expand our definition of the dialectical syllogism by saying this: the dialectical syllogism is a syllogism which produces an assent to one side of an issue, with a fear that the other side might be true.

We said before that through its form a syllogism always has some kind of certainty. But clearly opinion is not absolutely certain like reasoned out knowledge is. Therefore, it must be that the product of the dialectical syllogism is not completely certain because it uses matter, premisses, that are not completely certain.

Thus, we can also describe the premisses of a dialectical syllogism in a way that is parallel to the premisses of demonstration. We said before that premisses of demonstration are true, first and immediate, prior to and better known than the conclusion, and cause of the conclusion. The dialectical syllogism will not start from premisses that are true, immediate and first. Rather, it will start from probable premisses, premisses that lack certitude. In other words, the dialectical syllogism starts from opinions.

Aristotle describes the premisses of the dialectical syllogism as follows:

Syllogism, on the other hand, is dialectical, if it reasons from opinions that are probable. . . . Those opinions are probable which are accepted by everyone, or by the majority or by the philosophers.

Aristotle's explanation of the probable is that they are premisses accepted either by all men, or by most men, or by the wisest men. St. Albert points out to us in his commentary on Aristotle's Topics that when Aristotle defines the probable, he does not mean that various classes of people accept it. Rather, he also means that it is likely to be true. In fact, the word likely really means like the true. Various people accept various opinions because to them those opinions seem likely to be true.

We should now ask ourselves how we get the kinds of propositions that we can use as the premisses in the dialectical syllogism. He writes:

A dialectical proposition consists in asking something that is held by all men, or by most men, or by the wise.

As we saw before, the premisses of a dialectical syllogism must be probable, that is, held to be likely by all, by most, or by the wise. Since dialectic is an intellectual combat, the premiss most of all must be probable to the opponent. We find what is probable to our opponent by proposing a premiss in the form of a question. Such a question, which secures the premiss of a dialectical syllogism, is called the dialectical proposition.

The conclusion at which we are aiming also has a special name: it is called the dialectical problem. A problem because dialectic always presumes that there is a disagreement about what should be the conclusion of the dialectical inquiry. One side asks questions of the other hoping to lead him by syllogisms to assent to the questioner's side of the conclusion. The answerer tries to avoid letting that happen. Both hope to change the mind of the other about the dialectical problem.

For instance, two people might debate on this question of whether the world began in time. Suppose that the first holds that it had a beginning in time, and the second that it is eternal. The first might approach this dialectic problem as follows:

Dialectical problem: Did the world have a beginning in time? The dialectical problem about whether the world had a beginning in time has been resolved by syllogizing from premisses which are answers to the questions called dialectical propositions.

Q: The world was created by God, was it not? (First dialectical proposition)

A: Yes.

Q: But things that are made by another thing always have a beginning in time, are they not? (The second dialectical proposition)

A: I suppose so.

Q: Then it follows necessarily that the world had a beginning in time. (Conclusion which resolves the problem)

Now the subtleties of this dialectical art concern two things: how to find the desired dialectical propositions and how to avoid granting premisses. That is, the questioner has the task of finding dialectical propositions whose answers will lead to the conclusion he desires. The answerer must figure out how to avoid granting the premisses that lead to the conclusion that he opposes. The good dialectician knows both how to force his opponent to change his opinion and how to avoid letting his opponent force him to do so. The rest of Aristotle's Topics contains rules, called topics, which direct dialectical combat in these areas. Since discussing those rules in detail would constitute a course in itself (the Topics is the longest treatise in the Organon), we must be content in this course to examine only the basic nature of the dialectical tools.

The Second Tool: Induction

We have sufficiently discussed the dialectical syllogism. The next tool of dialectical reasoning that we must examine is induction. Aristotle discusses induction in the twelfth chapter of the first book of the Topics. He writes:

We must distinguish how many species there are of dialectical arguments. There is on the one hand induction, and on the other, syllogism. Now what syllogism is has been said before. Induction is a passage from individuals to universals. For example, the argument that supposing the skilled pilot is the most effective and likewise the skilled charioteer, then in general the skilled man is best at his particular task.

Aristotle here has given us both a definition and an example of induction. We are going to go over both, and then we are going to talk about the difference between induction and syllogism. Finally we are going to talk about the relation between induction and demonstration.

First, what is essential to induction is that it is a passage or reasoning process that goes from many individual cases to a universal case. Let us think about what is necessary in order to go from many individuals to the universal. First, since the statements about individuals lead to statements about universals, all of the individuals statements must have the same predicate. The statements Fido barks and Rover has four legs do not yield an induction because their predicates are difference. Fido barks and Rover barks do have a common predicate.

Second, there must be something common to all the individuals so that we can gather the statements about them into one universal statement. Rover is brown and the basketball is brown do not have a subject with a common nature. But Fido barks and Rover barks do contain subjects with a common nature. In sum, the individuals at the foundation of an induction must have a common nature that can be related to a common predicate.

Aristotle gives an example of an induction. All of the particular statements have a common predicate, most effective. All of the subjects, skilled pilot and skilled charioteer, have a common nature: skilled man. The induction concludes by predicating the common predicate of the common nature universally: Every skilled man is most effective. We can take our even more ordinary example. I see Fido bark, then I see that Rover barks, and finally I see Spot bark. All three have a common predicate: barking. There is a common nature to all three subjects, they are all dogs. From those three cases I then conclude, All dogs bark.

These examples should make clear the important difference between induction and the syllogism. With a syllogism, given that the premisses are true, the conclusion must be true. With an induction, even if all the premisses are true, the conclusion is not necessarily true. For example, in reality not every dog barks: the conclusion to the second induction turns out to be false. Therefore, in virtue of its form an induction only concludes with probability. That is why Aristotle speaks about induction primarily in the Topics, a book devoted to probable reasoning.

Even though induction does not conclude with certainty from its form, it remains a vital logical tool. For instance, many if not most of the great discoveries of modern science have been made using an inductive process, the careful observation of many different cases. Of course, in order to make those discoveries absolutely certain the scientists must move from induction to the demonstrative syllogism, but in order to come up with the discovery at all, that is, in order to have some reason to make that demonstration, the scientists first needs good reasons for thinking that his theory is probably true. Induction often provides those good reasons.

Furthermore, although induction does not achieve certainty through its form, sometimes it arrives at an absolutely certain conclusion because of its matter. In fact, the process by which we come to these first principles of demonstration is a process of induction.

Recall the example we talked about at the end of our last lesson. The doctor notices that when he gives a certain herb to Socrates, Socrates is cured of his fever. The same is true with Plato, and the same is true with Aristotle. And then he goes on to formulate the principle that this kind of herb always cures fever. This is a process of reasoning from individual statements to a universal, and thus it is clearly a process of induction. What Aristotle was teaching at the end of the Posterior Analytics was that induction is the way to the first principles.

We know, however, that those first principles are not just probable, they are certain. Thus before we conclude this lesson we should discuss how because of its matter an induction can yield a conclusion that is absolutely certain. An example is the process by which children come to understand shapes. You can give a child a toy that is a box with holes of various shapes in the sides, and pieces that can be put in. There is a square hole and a square piece, a round hole and a round piece, a triangular hole and a triangular piece. The child soon learns to put the triangular piece in the triangular hole. That is, he has grasped something about what a triangle is, even thought he has not formulated a definition of the triangle.

Suppose then that you take the child aside and start counting the sides of triangles. He says, This triangle has three sides, that triangle has three sides, this other triangle has three sides. The child will soon see that every triangle has three sides. Now the process of coming to that universal proposition is one of induction. But when he makes that jump to all triangles have three sides, he does not make only an induction. Rather, through the induction he comes to see more deeply into the nature of the triangle. He has come to see that having three sides is part of what it means to be a triangle.

In general, if we have the right matter, individuals which have a common nature and a common predicate which is part of the very definition of that common nature, then even though the form of the induction is uncertain, by the process of intellectual insight we can see from the matter of the induction that the conclusion has absolute certainty. The results of such inductions are the propositions that are the first principles of all demonstrative syllogisms.

St. Thomas calls such propositions self-evident. He defines the self-evident proposition as a proposition whose predicate is in the very definition of its subject. Since having three sides is in the very definition of triangle, it is self-evident that the statement, All triangles have three sides is true. Though the proposition is self-evident, we yet learn it through a process of induction.

This is how we can sum up what we have said about induction. Induction is a tool that primarily belongs to the process of dialectic. It is a tool by which the intellect goes from an understanding of many individual statements to an understanding of a statement about the universal. Unlike the syllogism, its conclusion does not necessarily follow from its premisses, though the conclusion is probably true given the truth of the premisses. On occasion, however, that induction can be the cause of someone coming to the insight that the predicate of the conclusion belongs to the very definition of the subject of the conclusion. In that case, the induction results in the person seeing that its conclusion is self-evidently true, and that kind of statement is a first principle of demonstrative science.


We have seen that dialectic is a power of intellectual combat whose primary tools or weapons are the dialectical syllogism and induction. Dialectic is about fair combat. There are no underhanded methods, no hidden tricks in dialectic. But there is a kind of intellectual combat which is unfair, in which underhanded tactics are used, in which hidden tricks are performed. The power to carry on this unfair intellectual combat is called sophistry. Aristotle talks about it in his book Sophistical Refutations, and its primary tool is the fallacy. In our next lesson we are going to look at Aristotle's Sophistical Refutations, the power of sophistry, and the kinds of fallacies.


1. Write a very short dialogue (about 1 page) in the style of Plato. Make sure the dialogue contains one induction and one dialectical syllogism.

2. Write a short essay explaining the dialogue. Point out the dialectical problem and the dialectical propositions. Then explain whether the dialogue would be suitable for intellectual training, causal intellectual discussion, or the philosophical sciences.


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