Deductive & Inductive Reasoning

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In logic, there are two broad methods of reasoning, deductive and inductive.


Slide Show, Critical Thinking Class, May 7, 2012


Some of the material in this PowerPoint presentation is already presented on this page, but here is a copy of the material as it was presented in class on Monday, May 7, 2012. If you're having trouble reading the text, clicking on the icon on the bottom right will give you a full-screen version.

Inductive Reasoning

Inductive reasoning moves from specific observations to broader generalizations. If you see one goose, you might think, geese are white, but you don't have a large enough sample to know. See two, and you begin to see a pattern, see the flock and you have a big enough sample to draw a conclusion: geese are white, or "most" geese are white.
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inductive reasoning, pattern recognition test


Sherlock Holmes, Silver Blaze


In the short story, The Adventure of Silver Blaze, Sherlock Holmes solves a murder by establishing the premise that dogs bark at strangers. Silver Blaze is a champion race horse who disappears from the stables the night his trainer is murdered. Holmes has two premises: 1) The dogs didn’t bark the night of the murderer broke into the barn where Silver Blaze was stabled, and 2) dogs bark at strangers. These premises lead Holmes to the conclusion the dogs knew the murderer. His premises turns out to be correct and lead to an accurate conclusion; Holmes solves the mystery and the murder. One could argue he's used deductive reasoning, setting up the general premise, "dogs bark at strangers," and then the more specific, "these dogs did not bark at whoever entered the stables," therefore, "whoever entered the stables was not a stranger." One could also argue Holmes is using inductive reasoning, essentially reasoning by sign, (the clues), and perhaps by cause, (because dogs bark at strangers, therefore...) In fact, Holmes uses both forms of reasoning to solve crimes.

Here's a clip showing the dogs barking at Holmes. It's from an old (1988) BBC version of the story. Here's a link for more Silver Blaze

Holmes' Hat Trick

Here's a clip from a Sherlock Holmes episode produced by the BBC in the 1980s. In it Holmes is establishing the character of a man he's never met simply by examining his hat and following the clues. Holmes basically uses Inductive reasoning to arrive at his conclusions. He establishes premises and then arrives at his conclusions based on them.


Owls: Inductive Reasoning

We can induce from these examples that owls (or most or many owls) have the ability to turn their heads in a unique way. We are reasoning from the one to the many.
Example #1: Owl turning its head
Example #2: Owl turning its head
Example #3: Owl turning its head

Cheetah Videos
Cheetah Girl video is a clip from France 3, a tv station in France
Cheetahs in Captivity
Cheethas in Slow Motion
Cheethas in the Serengeti, a documentary

Deductive Reasoning:

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Interestingly, the above reasoning about planets may not be sound. An asteroid in the solar system has been discovered revolving around the sun It's called Vesta and it's causing much debate among scientists. Check this out.
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Properly formed deductive arguments are called Valid Arguments The conclusion of a valid deductive argument only makes explicit what is already contained in the premises. In real life, of course, we can't always be sure of our premises, so the conclusions of real-life deductive arguments still have to be examined carefully.
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For the logic to be formatted correctly, we would have to say that some old TV shows are penguins. The category being defined is penguins. Penguins are many things beyond "black and white." Look to the first term.
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Men are many things beyond mortal. We don't know from the premise that all men are mortal, whether other creatures are mortal. We learn nothing about mortality from the first premise. We can not speculate about the mortality of Socrates because we don't know from the two premises whether he's a man. He could be the name of someone's dog. The second premise has to give us information about the category or quality we have explained.... in the above case that men are mortal.

How Penguins gave up flying.