Reasoning in Arguments

Joseph Reagle

Our question(s)

  • What are common types of reasoning?
  • What are some reasoning fallacies?

Argument & reasoning

Argument/reasoning/logic??

  • an argument is a claim or series of claims supported by evidence through reasoning
  • reasoning is drawing of a conclusion based on evidence and steps in thinking
  • logic is a formal structure for reasoning

Types of Reasoning

  • deduction (general to specific)
  • induction (specific to general)
  • abduction (inference to the best explanation, often by elimination)
  • analogical (similarity)

Structure of Reasoning

Type Premises Reasoning Quality Conclusion Truth
deductive true/false valid/invalid un/sound definite
inductive verified/representative strong/moderate/weak un/cogent probable
abductive exhaustive/incomplete best/competing un/warranted plausible
analogical relevant/irrelevant apt/superficial un/warranted plausible

Deductive

  • Reasoning from the general premises to specific conclusion.
  • For example: the syllogism, “Penguins are flightless birds; Bob is a penguin; thus, Bob is a bird that cannot fly.”
  • If premises are true and reasoning is valid, the conclusion is sound and definitely true.

Many everyday arguments leave premises unsaid (enthymeme)

Syllogism: formal deduction

an argument that consists of a major premise, a minor premise, and a conclusion drawn from those premises (Gunn 2017, loc. 467)

Major premise: “All men are mortal.”
Minor premise: “Socrates is a man.”
∴ Conclusion: “Therefore, Socrates is mortal.”

Induction

  • Reasoning from the specific to the general.
  • For example: Both Canada and Denmark have socialized medicine. They both also have high taxes. Therefore, if the US adopts socialized medicine our taxes will be increased. (Gunn 2017, loc. 466)
  • If premises are true and reasoning is strong, the conclusion is coherent and probably true.

Abduction

  • Reasoning by elimination.
  • For example: If parents have brown eyes and their three children have hazel eyes and we eliminate infidelity, it’s likely the parents are recessive hazel.
  • If eliminations are valid, the inference is plausible.

Induction & abduction

Analogical

  • Reasoning from similarity (can have elements of induction and abduction)
  • For example: learning a musical instrument is like learning a new language, requiring practice and immersion.
  • If the similarity is relevant and strong, the analogy is plausible.

Quiz

deductive, inductive, abductive, analogical?

  1. Training a cat is like training a dog, requiring patience and positive reinforcement.
  2. All birds have feathers. Penguins are birds. Therefore, penguins have feathers.
  3. The sun has always risen in the east every morning. Therefore, it will do so tomorrow.
  4. Cookies are missing, kids probably ate them.
  1. Analogical: cat ≈ dog

  2. Deductive: classic syllogism

  3. Inductive: past → future

  4. Abductive: the kids did do it before

Fallacies

Bad vegetables?

Ad Hominem

a fallacy that attacks the person rather than dealing with the real issue in dispute (Tucker 2018, p. 314)

example?

“Climate change is not true. It is supported by Congressman Jones, who was convicted of fraud last year.”

False cause

A correlation is used to imply causation.

example?

“Dinosaurs ate vegetables and went extinct; eating vegetables will kill us.”

Bandwagon / Ad Populum (“To the People”)

a fallacy that assumes that because something is popular, it is therefore good, correct, or desirable (Tucker 2018, p. 313)

example?

“Four out of five dentists would recommend chewing Cheesit gum!”

(also an “appeal to authority” fallacy)

Either-Or Fallacy / False dichotomy

a fallacy that forces listeners to choose between two alternatives when more than two alternatives exist (Tucker 2018, p. 313)

example?

“You can either have a successful career or be a good parent.”

Non Sequitur

the conclusion does not follow from its premise (Tucker 2018, p. 312)

example?

“All people are mortal; Socrates is a person; therefore, Socrates cannot dance.”

Begging the question

Circular reasoning wherein an argument’s premises assume the truth of the conclusion instead of supporting it. (Also known as “assuming the conclusion.”)

example?

“Marijuana should remain illegal because it’s a dangerous illegal drug.”

Red-Herring

creating a diversion or introducing an irrelevant point to distract someone or get someone off the subject of the argument (Tucker 2018, p. 314)

example?

“Other students cheat all the time and never get caught.”

(also false dichotomy)

Slippery Slope

something bad or undesirable will inevitably happen under a debated proposal

example?

“If we legalize marijuana, people will start using harder drugs, leading to widespread addiction and the collapse of society.”

Straw-man

a fallacy that shows only the weaker side of an opponent’s argument in order to more easily tear it down (Tucker 2018, p. 311)

example?

We need stronger regulations on industrial emissions.

That will shut down all factories and destroy the economy!

Conclusion

Wrap up

Find a “live” example of a fallacy on the web.

Review

Which fallacy?

Google “Recognizing Fallacies Exercises”, review their fallacy, and complete quiz.

  • Hasty generalization
  • False cause
  • False analogy
  • Ad hominem
  • Either-or
  • Bandwagon