A successful logical appeal provides evidence to support the overall argument.  If one fails to establish a logical appeal, then the argument will lack in both substance and sense.

For example, we can refer to the previous example of the politician’s speech writer to understand the importance of having a solid logical appeal.  What if the writer had only included the story about 80-year-old Mary without providing any statistics, data, or concrete plans for how the politician proposed to protect Social Security benefits? Without any factual evidence for the proposed plan, the audience would not have been as likely to accept his proposal, and rightly so.

Logic, Reason and Rationality

When an author relies on logos, it means that they are using logic, careful structure, and objective evidence to appeal to the audience. Objective evidence is anything that can be proven with statistics or other facts via more than one source. Oftentimes that evidence has been validated by more than one authority in the field of study.

For example, if Dr. Smith was trying to convince her students to complete their homework, she might explain that she understands everyone is busy and they have other classes (unbiased), but that completing their homework will help them get a better grade on their test (explanation). She could add to this explanation by providing statistics showing the number of students who failed and didn’t complete their homework versus the number of students who passed and did complete their homework (factual evidence). This is an example of logos employed for the purposes of argument and persuasion.

The use of logic, data, statistical evidence, and sufficient support to establish the practicality and rationality of your claims should be the strongest element of your argument. To have a logically sound argument, you should include:

• A debatable and supportable claim
• Logical reasoning to support your claim
• Sound evidence and examples to justify the reasoning
• Reasonable projections
• Concessions & rebuttals
• Avoid logical fallacies

Logical appeals rest on rational modes of thinking, such as the following:

• Comparison – a comparison between one thing (with regard to your topic) and another, similar thing to help support your claim. It is important that the comparison is fair and valid – the things being compared must share significant traits of similarity.
• Cause/effect thinking – you argue that X has caused Y, or that X is likely to cause Y to help support your claim. Be careful with the latter – it can be difficult to predict that something “will” happen in the future.
• Deductive reasoning – starting with a broad, general claim/example and using it to support a more specific point or claim (picture an hourglass where the sands gather in the middle)
• Inductive reasoning – using several specific examples or cases to make a broad generalization (consider the old question of “if your friend jumped off of a bridge, would you” to make the sweeping claim that all young people are easily persuaded to follow the crowd)
• Analogical reasoning – moves from one particular claim/example to another, seemingly sequential (sometimes this line of reasoning is used to make a guilt by association claim)
• Exemplification – use of many examples or a variety of evidence to support a single point
• Elaboration – moving beyond just including a fact, but explaining the significance or relevance of that fact
• Coherent thought – maintaining a well-organized line of reasoning; not repeating ideas or jumping around

Recognizing a Manipulative Appeal to Logos

Pay particular attention to numbers, statistics, findings, and quotes used to support an argument. Be critical of the source and do your own investigation of the facts.

Remember: What initially looks like a fact may not actually be one. Maybe you’ve heard or read that half of all marriages in America will end in divorce. It is so often discussed that we assume it must be true. Careful research will show that the original marriage study was flawed, and divorce rates in America have steadily declined since 1985 (Peck, 1993). If there is no scientific evidence, why do we continue to believe it? Part of the reason might be that it supports the common worry of the dissolution of the American family.

Using Visual Elements to Strengthen Logos

Adding visual elements to a persuasive argument can often strengthen its persuasive effect. There are two main types of visual elements: quantitative visuals and qualitative visuals.

Quantitative visuals present data graphically. They allow the audience to see statistics spatially. The purpose of using quantitative visuals is to make logical appeals to the audience. An appeal based on logic is called “logos,” and it persuades the reader using reasoning. Often, we can provide information in data form to persuade the reader through logic. Quantitative visuals help display the information clearly. For example, sometimes it is easier to understand the disparity in certain statistics if you can see how the disparity looks graphically. Bar graphs, pie charts, Venn diagrams, histograms, and line graphs are all ways of presenting quantitative data in spatial dimensions.