Unit 5 Fallacies

Learning outcomes

By the end of this unit you should:

  • be able to explain the differences between formal fallacies and informal fallacies
  • be able to describe rhetorical ploys and cognitive biases
  • have practiced identifying and naming fallacies

Introduction to fallacies

Activity 1 Reading

Read this introduction to understand the difference between formal and informal fallacies.

Fallacies are mistakes in the structure of an argument or reasoning used in an argument. These mistakes result in arguments that are invalid, and so the conclusions cannot be sound. In this course, we will divide fallacies into two broad categories, namely formal fallacies and informal fallacies. Formal fallacies are caused by problems in the structure of an argument. Informal fallacies are caused by problems in the reasoning, that is the jump from the premises to the conclusion. Rhetorical ploys are a type of informal fallacy which use emotion rather than reasoning. Cognitive biases are not fallacies per se, but are a faulty or mistaken way of thinking in which the conclusion is influenced by the individual. There are many different cognitive biases, One example of a cognitive bias is the confirmation bias. Confirmation bias is what happens when we select data, information or knowledge that confirms our current view, and so we do not consider the other choices carefully enough.

Activity 2 Watching

Consider watching the introductory videos to logic broadcast on NHK Eテレ: ロンリのちから [The power of logic] . These are in Japanese, so if you cannot understand Japanese, either learn it very quickly or skip this activity.


Activity 3 Reading

Read the texts below discussing monkeys and bananas. Identify which texts contain fallacies. For the texts that contain fallacies, decide whether the fallacies are formal or informal.

  1. Monkeys like yellow. Bananas are yellow. Therefore, monkeys like bananas.
  2. Monkeys like bananas. My mother said I was a little monkey. Therefore, I like bananas.
  3. Monkeys eat food. Bananas are food. Therefore, monkeys eat bananas.
  4. Bananas are yellow. Lemons are yellow. Therefore, bananas are lemons.
  5. Monkeys eat bananas. Monkeys are very strong. Therefore, eating bananas makes them strong.
  6. Monkeys live in trees. Bananas are in supermarkets. Therefore, monkeys go to supermarkets.
monkey eating banana

Activity 4 Reading

Read the texts below discussing aliens. Identify which texts contain fallacies. For the texts that contain fallacies, decide whether the fallacies are formal or informal.

  1. I have never seen an alien. Therefore, there are no aliens.
  2. There are many movies and books about aliens. If there were no aliens, there would be no films. Therefore, aliens exist.
  3. If there were no aliens, there would be no UFOs. Many people have seen UFOs. Since there are UFOs, there must be aliens.
  4. I am an alien, but on earth my body becomes human and I cannot change back into my usual green colour.
  5. Aliens are the life forms that evolved on other planets. There are other planets. Therefore, there may be other lifeforms.
  6. "We come in peace". Do not be afraid.
  7. Aliens have big heads. My teacher said I had a big head. Therefore, I am an alien.

Activity 5 Watching

Watch this video (3 min 41 sec). Which of the fallacies are formal and which are informal? Discuss your answers with a friend either face-to-face or online.

Activity 6 Thinking

Which of these are valid propositional forms and which are formal fallacies

  1. Timmy is either a girl or a boy. Timmy is not a girl. Therefore, Timmy is a boy.
  2. A monkey is a pet. A dog is a pet. Therefore, a monkey is a dog.
  3. If Timmy studies hard, he will learn. Timmy studies hard. Therefore, he will learn.
  4. Professional gamers play online games very well. Timmy plays online games very well. Therefore, Timmy is a professional gamer.
  5. My friend is called Timmy. My other friend's dog is called Timmy. Therefore, my friend is a dog.
  6. Dogs eat cats. Cats eat mice. Therefore, dogs eat mice.
  7. Ten million people bought XXX doggie biscuits for their dogs. Therefore, I should buy them, too.
  8. I passed all my exams so far by bringing my lucky teddy bear. I will bring my lucky teddy bear to the exam tomorrow. Therefore, I will pass the exam.
monkey dog
teddy bear

Knowledge and application

Activity 7 Video recording

Find or create a short video clip (max 2 mins) in Japanese or English that contains either a valid propositional argument or a fallacy. Label the clip with the name of the argument or fallacy. Submit the video clip through ELMS.

Don't let politicians deceive you, learn to spot fallacies!

Activity 8 Writing

Use the forum for Unit 5 on ELMS to discuss any of the arguments on this page. Some are valid, but many contain fallacies. Ask and answer questions. Post at least once question, and post at least one answer or response.


Make sure you can explain the following 6 concepts in simple English:

  1. fallacies: formal fallacies, informal fallacies, rhetorical ploys
  2. cognitive biases (e.g. confirmation bias)
  3. rhetorical devices, e.g. rhetorical ploy

Running count: 52 of 108 logical concepts covered so far.