Our first example, involving “mortal beings,” was an instance of the topic genus, which consists of all arguments made from the nature of a thing. It depends for its force upon the principle that there are fixed classes, and that what is true of a class may be imputed to every member of that class. In other words, any object will have the generic attributes of the class to which it belongs. Accordingly, if there is a genus “mortal beings,” and if “man” is a member of this genus, then man will have the attribute of mortality. The language is which the theory of this source of argument has been presented should not obscure the fact that genus is an exceedingly common source. Every time we hear a proposition such as “All Irishmen are brave” or “No politicians are honest” or “Some novels are romances,” we are encountering the topic genus, through which one object or class of objects is given, or denied, the attributes of another class.
For example, a teacher of French says to his class: “In writing French, never omit the accent marks. To a Frenchman accent marks are part of spelling.” This is an argument from definition clearly because accent marks have been defined as a part of spelling. To omit them therefore is the same as to leave something misspelled. This argument also implies a consequence, as do most arguments from definition. But as far as it goes, it says simply that the proper use of accent marks is one with correct spelling. Thus an argument for using French accent marks finds its support in a definition.
It may be helpful to recognize this source of argument in an historical context. Abraham Lincoln in many of his speeches against the spread of slavery put his case in the form of an argument from genus. In a speech made on October 16, 1854, for example, he reasoned as follows:
But if the Negro is a man, is it not to that extent a total destruction of self-government to say that he shall not govern himself? When the white man governs himself, that is self-government; but when he governs himself and also governs another man, that is more than self-government – that is despotism. If the Negro is a man, why then my ancient faith teaches me that ‘all men are created equal,’ and that there can be no moral right in connection with one man’s making a slave of another.
In this utterance it is clearly seen that Lincoln is employing “man” as a genus; that is to say, he is putting “man” in the same place in the argument in which we found “mortal beings” in the argument of our illustration. Now if it is conceded that the Negro belongs to the genus man, it has to be conceded that he has the attributes of manhood, one of which is the right to self-government. In brief, Lincoln is insisting that the attributes of the genus apply distributively to every member of the genus, and this is the rhetorical force of his argument.
It is necessary to understand that the success of this rhetoric depends upon a common acknowledgement of the genus “man.” If there is no common conviction that there is such a fixed genus, about which a definite set of predications can be made, then the argument must fail to exert rhetorical force. This fact will explain why many arguments contain lengthy definitions of their terms. Such definitions are nothing more than attempts to fix the genera which are to be used as terms in syllogistic reasoning. In the examples we have been studying, the genera are well established and accepted, and therefore there is little point in defining them. Obviously in most situations it would be needless or even absurd to define “man” or “mortal being.” But there are many genera whose attributes have never been carefully analyzed, and there are others which have been employed so much by pressure groups that the prevailing conception of them is a distorted one. When one is using a genus of this kind, one has no choice but to define or to redefine what he conceives it to be. When John Stuart Mill wrote his classic essay entitled “On Liberty,” he wished to establish certain conclusions about the liberty to which every individual is entitled. But realizing that the term itself has been so variously used by philosophers and by political partisans, he felt compelled to give a lengthy definition of it. Only after that definition was fixed could he use “liberty” in his argument with some hope of persuading people to agree with his conclusion.
The necessity of extended definition of a key term may be illustrated from contemporary usage. It would be a usual thing to hear someone observe, “This is the right course because this is the course of liberalism.” The argument as it stands is an enthymeme, which could be expanded as follows:
Now this argument will carry persuasive force if we know what liberalism is, and if we are for it. If we do not know what it is, naturally we cannot be for it, and in that case the argument would have no rhetorical impulsion. Therefore a minimum need would be to define liberalism, and since this word has suffered greatly from careless and partisan usage, we should take some pains with the definition. The same is at present true of the word “democracy,” despite some supposition that it is a universally understood term. It has been used to describe not only divergent but even contrary political conditions. If one used the word without defining it, one might find it creating an impulse quite different from what was intended.
Arguments from genus and from definition are the same in essence, since both of them are arguments from the nature of a thing, or from an established classification. In the case of the former, the classification is already established, or it is one of the fixed concepts in the mind of the audience to which the argument is addressed. In the case of the second, the work of establishing the classification must be done in the course of the argument, after which the defined term will be used as would any genus.
Arguments based on example belong to this group because an example always implies a general class. A genus must be involved because that is what the example is used to exemplify. It is the first step in an induction which, if carried out, would produce a generalization. When a speaker dwells on the fate of Napoleon at Waterloo, he is saying in effect: here is an instance of the truth that ambitious military conquerors finally overreach themselves and meet disaster. He may play up details of the event to make the instance vivid, but the force of the argument lies in the general proposition about military conquerors, which the example suggests.