Affirming the Consequent Fallacy: Definition & Examples

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  • 0:02 Up All Night
  • 0:43 Conditionals & Consequents
  • 1:36 Affirming the Consequent
  • 3:41 Lesson Summary
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Lesson Transcript
Instructor: Christine Serva

Christine has an M.A. in American Studies, the study of American history/society/culture. She is an instructional designer, educator, and writer.

Learn how to draw better conclusions by exploring the flaws in the affirming the consequent fallacy. You will be able to spot this problem even when the argument sounds logical.

Up All Night

Carla is very sensitive to caffeine. One cup of coffee in the evening and she'll be up all night long, unable to sleep. She always orders caffeine-free tea and soda. Plenty of times, she's ordered a caffeine-free drink and found herself jittery and lying awake in bed. When this happens, she concludes, 'They got my order wrong. It must be the caffeine keeping me up.'

In this lesson, we'll look at Carla's conclusion and consider whether she has reason to believe what she does about why she's up all night. We'll focus on the affirming the consequent fallacy and how to avoid confusing it with logic that is correctly used.

Conditionals and Consequents

Here's how Carla began her argument. She said, 'If I have caffeine in the evening, then I'm awake all night.' The format of her sentence is known as a conditional statement, an if-then statement which includes two parts: an antecedent and a consequent. The antecedent is the 'if' part of a conditional statement, and the consequent is the 'then' part of a conditional statement. Sometimes 'then' won't be used in the sentence, but the format is still basically 'If A is true, then C is true.'

There's nothing faulty in saying the statement that if she has caffeine, she'll be up all night. But then Carla goes on to say, 'I'm awake all night. Therefore, I must have had caffeine this evening.'

What's wrong with her logic? She's awake all night, so that's simply a fact she's reporting. Can Carla reasonably conclude that this means someone gave her a caffeinated beverage by mistake?

Affirming the Consequent

When Carla says, 'I'm awake all night,' she affirms the consequent has happened. She's awake. The fallacy of affirming the consequent occurs when a person draws a conclusion that if the consequent is true, then the antecedent must also be true.

Written in letters where the antecedent is represented by A, and the consequent is C, this argument looks like this: 'If A, then C.' 'C, therefore, A.' 'If I have caffeine' (antecedent), 'I will be awake all night' (consequent). 'I'm awake all night' (consequent). 'Therefore, I must have had caffeine' (affirms the consequent, concluding that the antecedent must have occurred).

This is not to be confused with the logical argument a person could make that goes like this: 'If I have caffeine' (antecedent), 'I will be awake all night' (consequent). 'I had caffeine. I will be awake all night.' In this case, it's okay to affirm the antecedent and then affirm that the consequent will then be true. This is because an if-then statement is designed to describe just such situations and gives you a logical argument to use. The problem is when the reverse occurs. Then a faulty conclusion could be made.

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