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Reliable facts can further strengthen your credibility and help your audience members make well-informed decisions—key effects of ethical public speaking. Sound reasoning that supports your claims is also essential if you hope to persuade audience members to change their beliefs or behaviors. When you present trustworthy facts to back your claims and clearly show how those facts have led you to those claims, you use logos effectively.

For example, suppose you want to deliver a persuasive speech arguing that diet soft drinks do not actually help people lose weight, a claim that might seem counterintuitive to many of your listeners. To convince them that you know what you’re talking about, you’ll need to supply proof, or evidence, of your claim. To further strengthen your logos, you’ll need to show that the conclusions you’ve drawn from the evidence make sense. Is your train of thought logical? Or are you using fallacious (faulty) reasoning to twist or distort the facts in your favor?

In the following sections, we discuss how to use evidence and reasoning to build a persuasive message and how to avoid several common logical fallacies.