Classic Flaws

If the sun comes out tonight, Lucy will go for a run. If Lucy goes for a run, Alexis will join her. Therefore, if Alexis and Lucy run together, the sun must have come out. And thus, if the sun does not come out tonight, Alexis will not run with Lucy.

Bad Conditional Reasoning

Classic Flaw where the author either concludes something by reading the conditional premises backwards without negating or negates the conditional premises and reads it forwards.

Loophole: what if we actually have to follow the rules of conditional reasoning?

Example: The contrapositive of this conditional statement is that “if Alexis does not join Lucy, Lucy will not run. If Lucy does not run, the sun must not have come out.” Alexis joining Lucy by itself does not tell us anything, the sun coming out is the sufficient and has to activate that necessary. We also don’t know anything about what happens when the sun does not come out.

Olivia is sad most of the time, and she also listens to Phoebe Bridgers every day, who makes sad music. Therefore, sad music must be responsible for Olivia’s sadness.

Bad Causal Reasoning

Classic Flaw where the author sees that two things are correlate and concludes that one of those things is causing the other.

Loophole: What if one of the Omitted Options is the case?

Example: We don’t have the evidence to prove that that’s what’s causing Olivia’s sadness. What if Olivia is sad for another reason and it makes her want to listen to Phoebe Bridgers’ sad music? What if Olivia just likes Phoebe Bridgers’ voice and it has nothing to do with her being sad? What if Olivia is losing her hearing, which is making her sad, and also means she can only listen to quieter music, which makes her listen to Phoebe Bridgers’?

Jessica just released a new book. Each chapter of the book discusses plants. Therefore, the book is about plants.

Jessica is writing a new book about plants. So, each chapter discusses plants.

Part-to-Whole & Whole-to Part

Classic Flaw where the author either says a member of a category has a property and uses that to conclude that the category itself also has that property or says a category has a property and uses that to conclude that a member of that category also has that property.

Loophole: What if wholes don’t necessarily equal parts?

Example: Just because the chapters each discuss plants doesn’t mean the whole book needs to be about plants. And just because she’s writing a book about plants doesn’t mean that each chapter has to talk about plants. We don’t have the evidence for that. She could be writing about plants in a book that’s actually about landscaping and she just mentions plants in each chapter. She could also be writing a book about plants but have a whole chapter on soil that doesn’t mention plants.

Kelly blow dries her hair at night. Last night, her blow dryer wouldn't turn on until she pressed the reset button. So all blow dryers require you to press the reset button.

Overgeneralization

Classic Flaw where the author talks about something having a property and then concludes that a bunch of other things also have that property.

Loophole: What if we can’t generalize from this one thing to a bunch of other things?

Example: Just because that’s how Kelly’s blow dryer works doesn’t mean it’s how everyone’s works.

Alyssa had to make a survey for her science class. She surveyed one person and found that 100% of respondents reported having allergies in the Spring. Therefore, all people have allergies in the Spring.

Survey Problems

Classic Flaw where the author concludes things based on a survey that has silent things wrong with it.

Loophole: What if the sample was biased, the questions were biased, there are other contradictory surveys, people lie on surveys, or the sample is too small?

Example: Alyssa only surveyed one person, so she can’t conclude anything from this survey.

Last year there was a study where college students were split into two groups. One group was given a metal water bottle and the other was given a plastic water bottle. The group with the metal water bottles drank double the amount of water as the group with the plastic water bottle. Therefore, metal water bottles are an incentive to drink more water.

False Starts

Classic Flaw where the author/researcher has a study with two groups and assumes the two groups are the same in all respects except those pointed out as part of the study, and then concludes that the differences in the study results are due to the one key difference the study is focusing on.

Loophole: What if the two groups were different in a key respect?

Example: this assumes that the difference in results is about the water bottle material, not the people. What if the metal water bottles went to people who already drink a lot of water? What if the metal water bottles went to people who live in a dorm that has a water bottle filler on every floor?

Melanie has a theory that trees blowing in the wind is a pleasant sound that can help people sleep. However, there are no studies that show people who sleep near trees blowing in the wind fall asleep easier. Therefore, the sound of trees blowing in the wind does not help people sleep.

Melanie has a theory that trees blowing in the wind can help people sleep. There are some studies that say tree blowing is a calming sound that can help people sleep. Therefore, Melanie's theory is correct.

Possibility ≠ Certainty

Classic Flaw where the author says that you can’t prove something is true so they assume it’s not true or that you can’t prove something is false therefore it can’t be false. Or, the author says that there is some evidence something is true so it must be true or that there is some evidence something is false so it must be false.

Loophole: What if lack of evidence ≠ evidence of lacking? What is proof of evidence ≠ evidence of proof?

Example: There not being evidence that tree blowing sounds help people sleep doesn’t mean that they don’t do that. Same goes the opposite way. There being some evidence that tree blowing sounds help people sleep doesn’t mean that Melanie’s theory is right.

Sarah thinks the color blue is really pretty on Michelle. Blue is the color that attracts bees the most. Therefore, Sarah thinks the color that attracts bees to Michelle looks pretty on her.

Sarah buys a blue shirt with the intention of getting Michelle to wear more blue. Therefore, Michelle is wearing more blue.

Michelle has started wearing more blue since Sarah bought her a blue shirt. Therefore, Sarah's purpose in buying the shirt was to get Michelle to wear more blue.

Implication (& Intention-Knowledge-Belief)

Classic Flaw where the author knows someone's intention, knowledge, or belief and uses that to proves something about what actually happened. Or, the author knows what actually happened and uses that to prove something about someone’s intention, knowledge, or belief.

Loophole: a mismatch between the premises and the conclusion. For Implication: What if the person in question isn’t aware of what their belief implies.

Example: First, what if Sarah had no idea that blue was the color that attracted bees. We can’t assume she knows the implication of the color blue. Second, just because Sarah intended to get Michelle to wear more blue doesn’t mean she wore more blue. Or, just because Michelle started wearing more blue doesn’t mean that was Sarah’s goal.