The ad hominem attack is the negative use of personal argumentation to undermine or destroy the credibility of a person in a critical discussion. An opposite type of tactic is the argument from expert opinion, which uses the opinion of a respected authority or expert on a subject as positive personal argumentation to support one's own side of an argument. The ad hominem criticism attacks a person as an untrustworthy source, while the argument from expert opinion cites an expert who is presumably reliable and authoritative as a source of advice. In certain respects however, these two types of argumentation are similar. Both are appeals to personal sources of opinion that center on the internal position or credibility of a particular individual as a reliable source of knowledge. Both types of argumentation can be contrasted with the appeal to external or objective knowledge, which comes from scientific evidence such as experimental observations, the kind of knowledge that comes from nature, not from a personal source.

In general, the use of argument from expert opinion is a reasonable, if inherently defeasible, type of argument. Appeals to expert opinion can be a legitimate form of obtaining advice or guidance for drawing tentative conclusions on an issue or problem where objective knowledge is unavailable or inconclusive. It is well recognized in law, for example, where expert testimony is treated as an important kind of evidence in a trial, even though it often leads to conflicting testimony, in a “battle of the experts.”

The purpose of this handbook is to furnish the reader with the basic methods of critical analysis of arguments, as they occur in natural language in the real marketplace of persuasion on controversial issues in politics, law, science, and all aspects of daily life. This is very much a practical (applied) subject, because each individual argument is, to some extent, unique. The technique of applying the general guidelines of criticism for each type of argumentation scheme to each individual case requires practical skills of good judgment and judicious interpretation in identifying the argument, and sorting out the main thread of the argument from the discourse it is contained in. These are pragmatic skills requiring prior identification of the type of dialogue in which an argument occurs.

Logical semantics is an important subject in its own right. It is the construction of consistent and complete theories based on semantical constants and the use of variables. Chapter 5 is about semantics, but the remaining eight chapters are mainly about the pragmatics of argumentation. For the most part, applying critical rules of good argument to argumentative discourse on controversial issues in natural language is an essentially pragmatic endeavor. It is a job requiring many of the traditional skills associated with the humanities: empathy, a critical perspective, careful attention to language, the ability to deal with vagueness and ambiguity, balanced recognition of the stronger and weaker points of an argument that is less than perfectly good or perfectly bad, a careful look at the evidence behind a claim, the skill of identifying conclusions, sorting out the main line of argument from a mass of verbiage, and the critical acumen needed to question claims based on expert knowledge in specialized claims or arguments.

One of the most common criticisms made in argument is the reply “That's beside the point!” or “That's irrelevant.” However, relevance is such a broad term that the criticism of being irrelevant could refer to many different kinds of failure or shortcoming in an argument. The study of relevance in argument begins by clarifying and classifying these different types of alleged failure that can prompt the criticism that a breach of relevance has been committed.

The primary basis of allegations of irrelevance stems from an important basic feature of all reasonable dialogue. Every argument presupposes a context of dialogue in which there is an issue, or perhaps several issues, being discussed. An issue means there is a proposition or question of controversy under discussion. Typically, an issue in dialogue suggests that there are two sides to the discussion. In other words, there is a certain specific proposition being discussed, and one participant in the dialogue is committed to that proposition being true while the other participant is committed to its being false. Of course, dialogues are not always this clear or simple, but when they are of this form, the type of dialogue may be called a dispute (or disputation). A dispute is a dialogue where one side affirms a certain proposition, and the other side affirms the opposite (negation) of that proposition.

The goal of this book is to help the reader use critical methods to impartially and reasonably evaluate the strengths and weaknesses of arguments. The many examples of arguments studied in this text are familiar, yet controversial specimens from such sources as political debates, legal arguments, international disputes on foreign policy, scientific controversies, consumer decision-making questions, ethical problems, and health issues. Any argument, including contexts of lively debate, conflict of opinion, reasoned persuasion, questioning, criticism or cross-examination, can be usefully analyzed by the methods that follow.

It is to be emphasized that the methods of this undertaking are essentially practical. They come as much or more under the topic of what is properly called logical pragmatics, as opposed to (semantical) logical theory. Logical theory traditionally has tended to emphasize semantic relationships, that is, relationships between sets of true or false propositions (the subject-matter of chapter 5 in this text). Logical pragmatics has to do with the use of these propositions by an arguer to carry out a goal of dialogue in reasoning with a second participant in the dialogue. One common and important type of goal is to successfully convince or persuade another arguer with whom the first arguer is engaged in reasoned dialogue. In logical theory, an argument is a set of propositions, nothing more or less. And all that matters is the truth or falsehood of these propositions. The wider context of dialogue is not taken into account.

The basic building blocks of arguments are propositions. Propositions, in contrast to questions, commands, challenges, and other moves made in arguments, are units of language that are true or false. Locating the propositions asserted in an argument can be the first positive step in identifying and evaluating the argument. In practice, real arguments are most often macrostructures made of many smaller arguments or sub-arguments. Often the best way to get a handle on a large and complex network of argumentation is to identify and clearly state one or more of these subarguments.

For this purpose, it is necessary to understand the concept of a deductively valid argument. With this ideal in mind, it will be much easier to organize an argument, and to fairly interpret it prior to considering evaluation. And of course, being able to identify deductively valid arguments is also a useful skill when it comes to evaluating arguments as reasonable or unreasonable.

In this chapter, we will see how once an argument is identified, the form of the argument can be revealed. Certain forms of argument are deductively valid, while others are invalid. Certain characteristic forms of deductive argument are very common in reasoning. To learn to identify these forms of argument is a valuable tool in the business of evaluating arguments reasonably. Another important concept related to validity is that of inconsistency. To allege that someone has adopted a stance in argument that is logically inconsistent can be a serious and damaging type of criticism.

Argumentation that takes place in the conversational marketplace of everyday persuasive appeals is heavily interlaced with emotional overtones and suggestions. Successful advertising, for example, seems for the most part to consist of well-orchestrated appeals to emotions, and it is quite plausible to suggest that many political debates and controversies are decided as much on the basis of emotional appeals and loyalties as on purely dispassionate reasoning.

Personal attack is often so successful as an argument tactic because of its hot appeal to personal emotions, as chapter 6 will indicate. However, here we turn to several other types of argument that have traditionally been regarded as problematic or fallacious because they use the pull of certain basic emotions. We will mainly be concerned with the emotions of pity, fear, and group solidarity.

Popular rhetoric is argument designed to persuade a specific target audience or readership. The objective is to build a personal bond with this audience, to establish a personal link between the arguer and the recipient of his message. The successful building of this emotional relationship invites the person to whom the argument is directed to trust the person who addresses him, to give him loyalty and to suspend the queries and criticisms characteristic of argument and reasonable dialogue. Personal rhetoric is therefore more directed to the instincts than to calculative reason. The emotional appeal is directed to the person's unthinking reactions, and so attempts to bypass the critical questioning and logical assessment normally characteristic of reasonable dialogue.

Normally in reasonable dialogue one is obliged to try to give a direct answer to a question, if one knows the answer, and if the question is reasonable and appropriate. If one does not know the direct answer, or for some reason cannot give it, then one is obliged to be as informative as possible. The reason behind this normal expectation is that our usual and reasonable presumption in many contexts is that a question is a sincere request for information where the questioner expects, or at least hopes, that the answerer may have this information and be able to give it. Therefore, if the answerer does not give a direct answer, his reply may be perceived as unhelpful or evasive.

Because of these normal expectations in reasonable dialogue, the most general purpose of a question is a request for information. Here, information refers to a set of propositions. So posing a question is a request to the answerer to supply a set of propositions.

There are several different types of questions each of which has a different format for requesting propositions. A whether-question poses a set of alternatives, and requests the answerer to select one. For example, the whether-question “Was she wearing the grey slacks or the red dress or blue jeans?” requests the answerer to pick one proposition from the disjunction. An example of a direct answer would be: “She was wearing the red dress.”

All the arguments and disputes we have been concerned with have been conducted and evaluated in the medium of natural language. But in natural language, words are vague and ambiguous. Words are most often not defined very precisely, and are therefore subject to the interpretation of the disputants in an argument. And since words in an argument may be interpreted in different ways, or according to different standards of precision, they can be used in a fashion that is friendly to the case of the arguer, and unfriendly to the case of the person to whom the argument is directed. Words can be used as weapons in argument.

When an Israeli border town is bombed, the newspapers in Israel describe the event as a terrorist attack, but Arab sources describe the same event as an action taken by freedom fighters in defence of their rights. However, when an Arab city is bombed, its inhabitants describe the event as a terrorist attack, unlike the Israelis, who describe it as a defensive strike against terrorists. The same group of individuals, in each instance, are described as “terrorists” by one side and “freedom fighters” by the other side. Now neither of these contentious terms has been defined, so they are subject to wide and various interpretations in a particular situation. For this reason, it is appropriate to question whether the two terms are being used in such a way that any action by the other side is routinely classified as that of “terrorists,” and any action by one's own side is classified as that of “freedom fighters.”

In a deductively valid argument, if the premises are true, the conclusion must be true. Deductive validity is a very strict standard of argument. If an argument is deductively valid, it is impossible for the premises to be true while the conclusion is false. In an inductively strong argument, if the premises are true, it is probable or likely that the conclusion is true. If an argument is inductively strong and the premises are true, it is logically possible that the conclusion could be false. So inductive strength is a less strict standard of argument than deductive validity. Inductive strength is a matter of probability.

Probability and statistics have an acknowledged place in scientific reasoning and experimental methods, but even outside these specialized contexts, the use of inductive argument is an important part of most reasonable dialogue. For example, the use of statistical arguments seems to play an increasingly significant role in political decision making on virtually any subject of discussion.

Of the many different kinds of inductive arguments, we will single out three for discussion in this chapter. An inductive generalization is an argument from premises about a specific group or collection of individual persons or things to a more general conclusion, about a larger group or collection. Traditional logic textbooks have often stressed the perils of hasty generalizations, for it has been rightly perceived that inductive generalization is associated with significant and common fallacies.

The argumentum ad hominem, meaning “argument directed to the man,” is the kind of argument that criticizes another argument by criticizing the arguer rather than his argument. Basically, this type of argument is the type of personal attack of an arguer that brings the attacked individual's personal circumstances, trustworthiness, or character into question. The argumentum ad hominem is not always fallacious, for in some instances questions of personal conduct, character, motives, etc., are legitimate and relevant to the issue. However, personal attack is inherently dangerous and emotional in argument, and is rightly associated with fallacies and deceptive tactics of argumentation. Three basic categories of fallacy have often traditionally been associated with three types of argumentum ad hominem.

The abusive ad hominem argument is the direct attack on a person in argument, including the questioning or vilification of the character, motives, or trustworthiness of the person. Characteristically, the focus of the personal attack is on bad moral character generally, or bad character for truthfulness.

The circumstantial ad hominem argument is the questioning or criticizing of the personal circumstances of an arguer, allegedly revealed, for example, in his actions, affiliations, or previous commitments, by citing an alleged inconsistency between his argument and these circumstances. The charge, “You don't practise what you preach!” characteristically expresses the thrust of the circumstantial ad hominem argument against a person.