To understand the full scope of argumentation, we must make a distinction between convincingand persuading. Although these words are often used interchangeably in ordinary speech, the student of rhetoric recognizes that to convince is not always to persuade. The term convince has a narrower meaning; it signifies forcing someone to accept a conclusion through the processes of valid argument. If people were changed in their attitudes and actions by being made to grasp the lines of an argument, the world might be a very different place, and we would no longer doubt whether or not man is a rational animal. But in truth the world is not governed by the syllogism, and everyone has witnessed the wisdom of the lines
A man convinced against his will
Is of the same opinion still.
We must not, of course, illogically jump from this to the conclusion that logic is useless. Logic is indispensable, but it will not accomplish everything. It has to be supplemented in most instances by an art to which the name “rhetoric” is sometimes specially applied. This art influences that part of our being not won over by pure reasoning. Although there have been many highly rhetorical discourses which were notable for their bad logic, rhetoric itself is not illogical. As Francis Bacon pointed out, rhetoric no more teaches men to make the worse appear the better cause than logic teaches them to reason fallaciously. Rhetoric is a means of persuasion which takes up where logic ends. After logic has convinced the mind, rhetoric wins the full assent. Therefore in looking at rhetoric, we turn our attention to what one does after making his logical argument.
First, however, we must pursue this distinction further by saying something about the difference between truth and validity. Most authorities on logic use the terms “valid” and “invalid” to indicate whether or not an argument is correct in form. It is essential to understand that these terms apply to the argument as a whole and not to its single propositions. A single proposition is true or false; an argument is valid or invalid. This is so simply because “valid” and “invalid” have references to form, and not to some kind of correspondence with reality, as do “true” and “false.” It is perfectly possible, therefore, to frame a valid syllogism having one or more false propositions. The only test of validity is correctness of form, as we saw in our study of the rules for deductive arguments. Consider for example the following syllogism:
Everyone knows that the major premise of this argument is false; nevertheless, if we accept the premise, the conclusion necessarily follows, and this is all that is required to make a valid syllogism. The purely formal nature of valid deductive argument can be seen strikingly if we make a syllogism with nonsense terms, such as the one below.
Or we can take a syllogism which consists of notation in the form of symbols.
Now it will be granted that these three arguments “convince” through their formal validity, but they do not persuade in the sense in which we are employing this term. We assent indifferently to the proposition “All S is P” because it does not refer to anything outside its system of symbolization. But as soon as we substitute for the symbols terms which have referents in the real world, our attitude changes. Suppose our syllogism now reads
This argument not only convinces us; it also persuades us, which is to say, it affects our attitude, or it initiates a course of action. In this case, it might be pointed out, one of the courses of action is to maintain the office of vice president. What has entered the argument to make this difference? The new factor is a statement about the world which we accept or reject according to our conception of what is true. Our interest is shifted here to the material truth of a proposition. We no longer ask merely “Is it in proper form?”; we ask also “Is it instantial?” which is to say, “Is it an instance of something factually true?’ The syllogism thus contains not only a valid structure of argument, but also an assertion about the world of reality which has some effect upon our impulse or action. This is where rhetoric appears to supplement logic. Logic addresses itself to the mind; rhetoric addresses itself to the total being, which includes the will. Man is not merely a thinking machine, which is why it is seldom if ever sufficient to give him a conclusion punched out in a machine-like operation. Consequently a complete argument, in addition to obeying the laws of thought, must embody something that we recognize as true about existence. We will be swayed by the argument according to our estimate of the truth of the assertion.
Our next step, therefore, is to discuss where the rhetorical impulse of any argument is to be sought. If our argument is to have more persuasive power than “All arps are wackles,” then a true or probably true perception about the world as we know it must be present.
Now as we return to our syllogism which has real terms and read it searchingly for the kind of predication that it makes about the world, we see that the middle term “man” relates the minor term “President” to the major term “mortal beings.” Thus the argument runs that if the President belongs in the class “man” and if all men belong in the class “mortal beings,” then the President too belongs in the class “mortal beings.” In other words, the President has the generic attribute “mortal being.” In this way the argument depends for its rhetorical force upon the acceptance of the class “mortal beings” as a class having a certain attribute (in this case the certainty of death) which can be ascribed distributively to every member of the class.