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The persuasive power of any speech is based on the quality of the arguments within it. Arguments themselves are comprised of three elements: claims, evidence, and warrants.
Source: Robert H. Lustig, Laura A. Schmidt, and Claire D. Brindis, “Public Health: The Toxic Truth About Sugar.” Nature 482 (February 2, 2012), doi:10.1038/482027a.
Identify the Nature of Your Claims
Depending on the nature of the issue, an argument may address three different kinds of claims: of fact, of value, and of policy. Each type requires evidence to support it. A persuasive speech may contain only one type of claim or, very often, consist of several arguments addressing different kinds of claims. In a persuasive speech, a claim can serve as a main point or it can be the speech thesis (if using only one claim).
Checklist: Structure the Claims in Your Persuasive Speech
When addressing whether something is or is not true, or whether something will or will not happen, frame your argument as a claim of fact.
When addressing issues that rely upon individual judgment of right and wrong for their resolution, frame your argument as a claim of value.
When proposing a specific outcome or solution to an issue, frame your argument as a claim of policy.
Use Convincing Evidence
Every key claim must be supported with convincing evidence, supporting material that provides grounds for belief. Chapter 8 describes several forms of evidence: examples, narratives, testimony, facts, and statistics. These most common forms of evidence—called “external evidence” because the knowledge does not generate from the speaker’s own experience—are most powerful when they impart new information that the audience has not previously used in forming an opinion.9
You can also use the audience’s preexisting knowledge and opinions—what listeners already think and believe—as evidence for your claims. Nothing is more persuasive to listeners than a reaffirmation of their own attitudes, beliefs, and values, especially for claims of value and policy. To use this form of evidence, however, you must first identify what the audience knows and believes about the topic, and then present information that confirms these beliefs.
Finally, when the audience will find your opinions credible and convincing, consider using your own speaker expertise as evidence. Be aware, however, that few persuasive speeches can be convincingly built solely on speaker experience and knowledge. Offer your expertise in conjunction with other forms of evidence.
Address the Other Side of the Argument
A persuasive speech message can be either one- or two-sided. A one-sided message does not mention opposing claims; a two-sided message mentions opposing points of view and sometimes refutes them. Research suggests that two-sided messages generally are more persuasive than one-sided messages, as long as the speaker adequately refutes opposing claims.10
All attempts at persuasion are subject to counterargument. Listeners may be persuaded to accept your claims, but once they are exposed to opposing claims they may change their minds. If listeners are aware of opposing claims and you ignore them, you risk a loss of credibility. Yet you need not painstakingly acknowledge and refute all opposing claims. Instead, raise and refute the most important counterclaims and evidence that the audience would know about. Ethically, you can ignore counterclaims that don’t significantly weaken your argument.11
Use Effective Reasoning
Reasoning is the process of drawing conclusions from evidence. Arguments can be reasoned inductively, deductively, or causally. Arguments using deductive reasoning begin with a general principle or case, followed by a specific example of the case, which then leads to the speaker’s conclusion.
In a deductive line of argument, if you accept the general principle and the speaker’s specific example of it, you must accept the conclusion:
GENERAL CASE (“MAJOR PREMISE”): | All men are mortal. |
SPECIFIC CASE (“MINOR PREMISE”): | Socrates is a man. |
CONCLUSION: | Therefore Socrates is mortal. |
Reversing direction, an argument using inductive reasoning moves from specific cases (minor premises) to a general conclusion supported by those cases. The speaker offers evidence that points to a conclusion that appears to be, but is not necessarily, true:
SPECIFIC CASE 1: | In one five-year period, the average daily temperature (ADT) on Continent X rose three degrees. |
SPECIFIC CASE 2: | In that same period, ADT on Continent Y rose three degrees. |
SPECIFIC CASE 3: | In that same period, ADT on Continent Z rose three degrees. |
CONCLUSION: | Globally, average daily temperatures appear to be rising by three degrees. |
Arguments based on inductive reasoning can be strong or weak; that is, listeners may decide the claim is probably true, largely untrue, or somewhere in between.
Reasoning by analogy is a common form of inductive reasoning. Here, the speaker compares two similar cases and implies that what is true in one case is true in the other. The assumption is that the characteristics of Case A and Case B are similar, if not the same, and that what is true for B must also be true for A.
Arguments can also follow lines of causal reasoning, in which the speaker argues that one event, circumstance, or idea (the cause) is the reason (effect) for another. For example, “Smoking causes lung cancer.” Sometimes a speaker can argue that multiple causes lead to a single effect, or that a single cause leads to multiple effects. (For more details on the cause-effect pattern, see Chapter 13.)
Avoid Fallacies in Reasoning
A logical fallacy is either a false or erroneous statement or an invalid or deceptive line of reasoning.12 In either case, you need to be aware of fallacies in order to avoid making them in your own speeches and to be able to identify them in the speeches of others. Many fallacies of reasoning exist; the following are merely a few.
LOGICAL FALLACY | EXAMPLES |
Begging the questionAn argument that is stated in such a way that it cannot help but be true, even though no evidence has been presented. |
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Bandwagoning An argument that uses (unsubstantiated) general opinion as its (false) basis. |
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Either-or fallacy An argument stated in terms of only two alternatives, even though there may be many additional alternatives. |
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Ad hominem argument An argument that targets a person instead of the issue at hand in an attempt to incite an audience’s dislike for that person. |
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Red herringAn argument that relies on irrelevant premises for its conclusion. |
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Hasty generalizationAn argument in which an isolated instance is used to make an unwarranted general conclusion. |
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Non sequitur (“does not follow”)An argument in which the conclusion is not connected to the reasoning. |
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Slippery slopeA faulty assumption that one case will lead to a series of events or actions. |
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Appeal to traditionAn argument suggesting that audience members should agree with a claim because that is the way it has always been done. |
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