Using Appeals in Persuasive Speeches: Logos

1. Evidence or Logic (Logos)

Evidential appeals are formed by defining the evidence and then explaining how the evidence must logically prove that a certain conclusion must be true. They are the only type of persuasive speech allowed in a court of law; the evidence must prove that the defendant has committed the crime in order for that person to be found guilty. Evidential appeals are also the basis for scientific research. A scientist must be able to show the connection between evidence and a conclusion in order for their work to be accepted.

In persuasive speaking, the speaker must first explain the evidence in a way that is comprehensible to the audience, yet complete. Then, the scientist must explain how that evidence logically leads to a consequence that supports their proposal.

In the context of persuasive speaking, evidence can be evaluated for its persuasive ability in the following three ways:

1. Accuracy: The evidence must be truthfully constructed and defined. For example, if an argument hinges on the premise that grass is purple, no rhetorical technique will be able to persuade the audience. Evidence must be accurate to be credible, as its credibility rests on its accuracy.
2. Audience understanding: Evidence must be presented completely but in a manner that the audience can comprehend. For example, an evidential appeal that uses rising carbon dioxide levels as evidence for stricter pollution regulation will not be effective if the audience does not know what carbon dioxide is or why it is bad for the environment. The audience must be able to understand the evidence before it is used in an appeal.
3. Relevant context: The evidence must be defined within the context of the appeal. A textbook definition of the different types of bonds between the atoms of carbon dioxide is not relevant information for why the tax rate should be lowered.

terms to know

Evidential Appeal

An attempt to show the logical connection between a set of evidence and a consequence. Also known as logical appeal or logos.

Evidence

The available body of facts or information indicating whether a belief or proposition is true or valid.

2. Strategies for Deploying Evidence Effectively

When using evidential appeals in your persuasive speech, make sure to deploy evidence strategically in order to argue your point. As you craft your persuasive speech, ask yourself the following questions:

1. Is my evidence accurate? Make sure that your evidence, be it facts, statistics, personal testimony, or other pieces of information, comes from credible sources. Credible sources are well-known, respected leaders or sources of information in a given field. Make sure the source is impartial and unbiased. It's also helpful to anticipate any questions your audience may have about your evidence, including any challenges they may make to its credibility or accuracy.
2. Is my evidence relevant? How well does your evidence actually back up your argument? Just because it might be believable to you does not mean it will be as compelling to another. That disconnect might occur when your evidence is not actually relevant to your argument. Make sure your evidence is directly related to the points you are trying to make. Likewise, your audience may have evidence of their own to contradict your line of reasoning; anticipate these contradictions and argue your point with evidence to counter their disagreements.
3. Is my evidence thorough? Sometimes your case may not be compelling simply because you haven't fully supported your thesis. Your evidence may only skim the surface. It's a fine balance between thoroughly explaining and defining your evidence to your audience and overwhelming them in the process. In this regard, it's important to have a comprehensive understanding of the knowledge base of your audience. Let the lowest level of understanding in your audience dictate how thorough you need to be; at the same time, don't overexplain or get into so much depth that you lose more of your audience than you win over.

terms to know

Accuracy

Exact conformity to truth, or to a rule or model; degree of conformity of a measure to a true or standard value.

Thorough

Painstaking and careful not to miss or omit any detail.

3. Ethical Usage: Considering Other Viewpoints

The honest consideration of other viewpoints is an ethical duty if you are a persuasive speaker seeking to convince the audience of something you believe to be true.

You may be making the wrong conclusion based on the evidence, or your evidence may be flawed, both of which can be shown by examining other views. Perhaps you and your opponent are using the same evidence but come to different conclusions. If you are able to consider other viewpoints and still believe in your original view, then you are ethically able to attempt to persuade others.

As a persuasive speaker, there is always an incentive to invent or even just fudge evidence so as to strengthen your appeal and weaken opposing viewpoints. However, this is akin to lying and clearly unethical.

Furthermore, the evidence you provide must generally be accepted only after intense scrutiny. Statistics, and many types of evidence, do not fall neatly into the category of "true" or "false." While there are some undeniable truths, such as the fact that the earth is round, there are many more that fall into a gray area. This is due to the fact that evidence comes from a process that may be flawed. When the process is flawed, there may be a statistical fallacy.

EXAMPLE

If a survey asks skewed questions, the results may not reflect the truth. Though many researchers, scientists, pollsters, and investigators do their best to avoid fallacies, the possibility always exists that one may be proven.

term to know

Fallacy

An error in reasoning often due to a misconception or a presumption; used in informal discourse to mean an argument that is problematic for any reason.

4. Different Lines of Reasoning

Reasoning is the means by which rational beings understand cause and effect, truth and falsehood, validity, and what is good or bad. The result is a reason that could then be used to explain or justify some event, phenomenon, or behavior.

As you develop arguments for your persuasive speech, you are likely to engage in two different lines of reasoning: inductive and deductive.

4a. Inductive Reasoning and Associative Reasoning

Inductive reasoning is a kind of reasoning that constructs general propositions that are derived from specific examples based on previous observations. One important aspect of inductive reasoning is associative reasoning: seeing or noticing similarities among the different events or objects that you observe.

EXAMPLE

Here is a statistical syllogism to illustrate inductive reasoning:

1. Ninety percent of humans are right-handed.
   
2. Joe is a human.
   
3. Therefore, the probability that Joe is right-handed is 90%. If you were required to guess, you would choose "right-handed" in the absence of any other evidence.

EXAMPLE

Here is another stronger example:

One hundred percent of life forms that we know of depend on liquid water to exist. Therefore, if you discover a new life form, it will probably depend on liquid water to exist.

This argument could have been made every time a new life form was found and would have been correct every time. While it is possible that in the future, a life form that does not require water will be discovered, in the absence of other factors, the conclusion is probably correct, as it has been in the past.

terms to know

Inductive Reasoning

A kind of reasoning that constructs or evaluates general propositions that are derived from specific examples. Inductive reasoning contrasts with deductive reasoning, in which specific examples are derived from general propositions.

Syllogism

An inference in which one proposition (the conclusion) follows necessarily from two other propositions, known as the premises.

4b. Deductive Reasoning

Deductive reasoning is the process of reasoning from one or more general statements, laws, or principles regarding what is known, in order to reach a logically certain conclusion. Deductive reasoning involves using given, true premises to reach a conclusion that is also true. If you accept or know the general principle as true, then you can apply it to the specific case to conclude that it is also true.

EXAMPLE

Consider the general principle of the law of gravity: What goes up must come down. Now, when you throw the ball in the air, you conclude that it will fall down based on your knowledge of the general law of gravity.

Deductive reasoning contrasts with inductive reasoning in that a specific conclusion is arrived at from the general principle when reasoning deductively. If the rules and logic of deduction are followed, this procedure ensures an accurate conclusion.

EXAMPLE

Here is a classic example of a deductive argument:

1. All men are mortal.
   
2. John is a man.
   
3. Therefore, John is mortal.

The first premise states that all objects classified as "men" have the attribute "mortal." The second premise states that "John" is classified as a "man"—a member of the class or group of "men." The conclusion then states that "John" must be "mortal" because he inherits this attribute from his classification as a "man." If both premises are true, the terms are clear, and the rules of deductive logic are followed, then the conclusion of the argument follows by logical necessity.

term to know

Deductive Reasoning

The process of reasoning that uses given true premises to reach a conclusion that is also true. Deductive reasoning contrasts with inductive reasoning.

5. Deploying a Rational Appeal

A rational appeal uses logical arguments and factual evidence to persuade individuals that whatever thesis you are supporting is viable and likely to result in the obtainment of goals. The burden of proof is on you as the speaker as you develop your appeals to the audience.

Prior to your speech, it is important to consider the soundness of your evidence and reasoning.

• Deductive reasoning: If you are engaging in deductive reasoning, you will want to consider whether or not the audience is likely to accept the general premise as valid and true before you attempt to deduce other ideas or courses of action based on the general premise. If you are quoting an authority and drawing conclusions from the authority, it is important to ask if the audience will accept or believe the authority. Remember to quote or use sources that the audience is familiar with and will believe; using other authorities or sources will likely not be productive.
• Inductive reasoning: If you are engaging in inductive reasoning, you will want to consider whether you have observed or collected enough evidence to draw a highly probable conclusion. Or did you draw a hasty conclusion based on too few examples or observations? If you are using statistical evidence as part of your inductive reasoning, it is important to consider how the data was collected and whether it is truly valid. If you do not have valid statistical data, then the inductions will not be valid.
• Associative reasoning (analogy): When engaging in associative reasoning, you will want to make sure that the ideas are indeed similar and that there are no obvious or outstanding differences that would negate the association in the mind of your audience.

6. Logical Fallacies

A logical fallacy is an error in reasoning. There are two basic categories of fallacies:

• Formal
• Informal

6a. Formal Fallacies

Formal fallacies occur when there is a problem with the form, or structure, of the argument. "Formal" refers to the form of the argument. An argument that contains a formal fallacy will always be invalid.

EXAMPLE

Consider an example with a visualization of faulty reasoning involving categorical deduction:

1. All flowers are animals.
   
2. All animals can jump.
   
3. Therefore, all flowers can jump.
   

Even though it is quite obvious that the first premise is not true and further that the conclusion is not true, the whole syllogism is still valid. By applying formal logic to the syllogism in the example, the conclusion is still valid.

6b. Informal Fallacies

An informal fallacy is an error in reasoning that occurs due to a problem with the content, rather than mere structure, of the argument. In informal logic and rhetoric, a fallacy is usually an error in reasoning, often due to a misconception or a presumption. Some of the more frequent common logical fallacies are listed in the following table:

terms to know

Red Herring

A clue or information that is intentionally or unintentionally misleading, that diverts attention from a question; often thought to relate to using smelly fish to train dogs to recognize the real scent of something they were supposed to be tracking.

Straw Man

An insubstantial concept, idea, endeavor, or argument, particularly one deliberately set up to be weakly supported, so that it can be easily knocked down, especially to impugn the strength of any related thing or idea.