I leave it to you now and come back to the basic question which I raised with you: what is the need of an argument? How do you formulate an argument and how do you evaluate an argument? These are the three things that every student of philosophy should learn. My first answer is: you need to argue because even though we are sure that there are events in the world; there are phenomena in the world, our perception of these data is limited, and different individuals perceive different data and each one claims that his perception of data is obvious. And therefore, when they sit in an assembly like ourselves now, there is this comparison of data and while comparing the data there is the birth of an argument.
These arguments are of different kind. This argument of Parmenides is what is called a logical argument. There can be many other kinds of arguments, but this one is a logical argument. In larger terms it is called an epistemological argument. You remember I told you sometimes ago of the study of epistemology. These words don't worry if you don't remember, because you will come across again and again and in any case will learn them. So don't try to memorise them. Each time I shall explain these terms because they need to be repeated. Epistemology is a study of the means of knowledge, of the nature of knowledge, of the standard of knowledge, of criteria of knowledge. It is a study of why we call something knowledge at all. How do we decide this is knowledge? What are the conditions that should be fulfilled for any statement to be called a statement of knowledge? If a statement consists of an error surely we will not call it knowledge. There must be standards by which you can say that this statement is to be called knowledge. It must satisfy certain criteria. The study of this domain is called epistemology.
Logic is a part of epistemology. Why? Because logic states that thinking or thought is one of the processes of knowledge. You arrive at knowledge by a process of thinking. That is a claim, whether it is right or wrong is a different matter. The claim is that thought is a means of knowledge. And thought, if it follows certain criteria, is bound to give a correct knowledge, without error. That is the study of Logic. Logic says that thought, if it follows certain standards, is bound to be correct. Because correct, therefore knowledge. I had given you an example earlier, a very simple example: "All planets rotate around the sun". This is a statement which we know is valid; because it is a fact, let us say it is obvious. Now you reverse it: "Therefore all that rotates around the sun are planets". Is it a correct statement? No. Why, because this does not follow the criteria of correct thinking. You are reversing the statement; in logic it is called the process of conversion. A statement is given, then you convert it, that is to say you make it reverse, as I did just now. "All planets rotate around the sun; therefore all that rotates around the sun are planets." This statement is not correct; there is an error in it. What is the error? Logic points out what is the error in this kind of conversion. I shall tell you afterwards what is the error exactly but we know that it is not correct because there are comets also which rotate around the sun. Therefore it is a fact that this statement is not correct.
So, logic is a science in which criteria are lay down as to when a process of thinking is correct or when it is not correct. All men are mortal - convert it - all mortals are men. Is it correct? No. There is something in this conversion which has gone wrong. Logic finds out what is wrong in it. Very often when people argue heatedly they make this kind of mistake. They just put one statement, then they reverse it. And they try to prove to you their statement is true. But if you are a good logician, a good student of epistemology you will say: "Please wait, don't be heated, argue very quietly, let us see your statement, we shall examine whether your argument is valid or not." So, converting a statement is also called an argument. Making a statement and converting it is an argument. An argument is manipulation of thought which deserves to be examined. This is the meaning of an argument. You start with process of thinking and then you manipulate that process of thinking. You can manipulate any process of thinking in many ways. A is equal to B, B is equal to C, C is equal to D, D is equal to E, E is equal to F, therefore A is equal to F. You are manipulating the whole process of thinking. There are many other ways of manipulating thinking. Epistemology or logic tells you whether that manipulation can be properly examined, and after examination you can arrive at a judgment whether it is without error or there is error in it. An argument is a manipulation of a thought process which deserves to be examined, which is to be tested.
Now this particular statement that I made from Parmenides, is also a kind of manipulation of thought process. Parmenides found out that all process of thought, whenever you think, always has an object. Thought does not exist without an object. It is one conclusion, one obvious proposition, which has been found out by Parmenides. It is on that basis that this argument is now formulated. And you examine it, and the peculiarity of this examination is, that the moment you examine it, it is asserted. To examine you have to say: "No this is not true". Starting point of examination is "This is not true". If it is not true, then you have to find a thought which has no object. But the moment you think there is always an object. Therefore this statement is reasserted by itself. And this is one of those prized arguments in the history of philosophy. You will find this kind of argument repeating itself in different forms, but the same argument comes again and again. As I told you in the twentieth century Bertrand Russell at the end of the examination of this argument says "I postpone this examination".
Even a dialectic argument is logical in character, but not vice-versa. All dialectical arguments are logical, but all logical arguments are not necessarily dialectical. So, there is a difference between the two. All arguments, in a certain sense you might say, not only dialectical argument, all arguments in philosophy are logical or they ought to be logical, but therefore all logical arguments are not necessarily dialectical.