By Tajammal MaqboolFounder & Developer
Learn to distinguish between arguments that fail because of broken logical structure and those that fail because of misleading content or irrelevant premises. This distinction is crucial because the two types require different diagnostic strategies. Formal fallacies can be detected by examining abstract structure alone, while informal fallacies require evaluating the relevance and truth of the content.
Fallacies fail in two different ways, and mixing them up is a common mistake. A formal fallacy has a broken shape: 'all dogs are mammals; all cats are mammals; so all cats are dogs' fails no matter what words you use. An informal fallacy has a fine shape but rotten content: an irrelevant reason, a slippery word, a hidden assumption. This exercise drills telling which kind of defect is doing the damage.
Some arguments fail because their structure is broken and others because their content is misleading, and the distinction changes how you respond. This exercise trains you to tell a defective argument shape from a valid shape carrying an irrelevant or false premise, which is the first step in any fallacy diagnosis.
Formal fallacies are almost mechanical: swap the nouns for letters and the broken structure stays broken. The two you'll see most are assuming the cause from the effect, and denying the effect to deny the cause. Both look like the valid forms but aren't. If replacing the words keeps it broken, it's formal.
Informal fallacies don't share one structure. They're a grab-bag grouped by how they fail: irrelevant reasons, question-begging assumptions, or slippery meanings. The practical payoff is in the response: formal fallacies you defuse by exposing the structure; informal ones by addressing the content. For deeper treatment, see Logical Fallacies: Formal & Relevance.
Question 1
Consider two flawed arguments: (A) "All dogs are mammals. All cats are mammals. Therefore, all cats are dogs." (B) "You should not believe Dr. Rivera's research on nutrition because she is overweight." What best describes the difference between these two errors?
Question 2
A manager reasons: "If an employee is consistently late, they are not committed to the job. Jordan is extremely committed to the job. Therefore, Jordan is never late." This reasoning feels intuitive but contains a formal fallacy. Which one?
Question 3
A parent tells their teenager: "If you study hard, you will get into a good college. You did not study hard. So you will not get into a good college." The teenager feels this is unfair. Is the teenager's intuition correct, and if so, why?
Question 4
A real estate listing reads: "This charming home is in a vibrant neighborhood with easy access to nightlife." A buyer later complains that "charming" meant small and run-down, and "vibrant with nightlife" meant loud and rowdy. What type of fallacy is at work in the listing's language?
Question 5
Someone argues: "The scientific establishment was wrong about ulcers being caused by stress. It turned out to be bacteria. They were wrong about dietary fat causing heart disease, because the sugar industry manipulated the research. So why should we trust what scientists say about climate change?" Which category of fallacy does this best fit?
Question 6
A colleague argues: "The project management app says our team velocity is 42 story points. The marketing team's velocity is 38. So our team is more productive." You sense something is wrong but cannot immediately articulate it. What makes informal fallacies like this one harder to detect than formal fallacies?
Where to go after this exercise.
