Deduction:Deduction: reasoning from general premises, which are known or presumed to be known, to more specific, certain conclusions.

Induction:Induction: the process of observing data, recognizing patterns and making a conjecture of your observations.

Both deductive and inductive arguments occur frequently and naturally…both forms of reasoning can be equally compelling and persuasive, and neither form is preferred over the other (Hollihan & Baske, 1994).

Deduction:Deduction: commonly

associated with “formal logic.”

involves reasoning from known premises, or premises presumed to be true, to a certain conclusion.

the conclusions reached are certain, inevitable, inescapable.

Induction:Induction: commonly known as

“informal logic,” or “everyday argument”

involves drawing uncertain inferences, based on probabilistic reasoning.

the conclusions reached are probable, reasonable, plausible, believable.

Deduction It is the form or

structure of a deductive argument that determines its validity

the fundamental property of a valid, deductive argument is that if the premises are true, then the conclusion necessarily follows.

The conclusion is said to be “entailed” in, or contained in, the premises. example: use of DNA

testing to establish paternity

Induction By contrast, the form or

structure of an inductive argument has little to do with its perceived believability or credibility, apart from making the argument seem more clear or more well-organized.

The receiver (or a 3rd party) determines the worth of an inductive argument

A sample of fifty motorists who were stopped by the CHP at a sobriety checkpoint on a Saturday at midnight revealed that one in four drivers were either uninsured, intoxicated, or both. Thus, if you get involved in an accident on the freeway there is a 25% chance the other motorist will be drunk or uninsured.

The Law of the Sea treaty states that any vessel beyond a 12 mile limit is in international waters. The treaty also states that any vessel in international waters cannot be legally stopped or boarded. Therefore, when the U.S. Coast Guard intercepts boats coming from Cuba or Haiti more than 12 miles from the U.S. coast, it is violating the Law of the Sea.

Example of Deduction

major premise: All tortoises are vegetarians

minor premise: Bessie is a tortoise

conclusion: Therefore, Bessie is a vegetarian

Example of Induction

Boss to employee: “Biff has a tattoo of an anchor on his arm. He probably served in the Navy.”

Bessie

tortoisesvegetarian animals

All tortoises fall in the circle of animals that are vegetarians

Bessie falls into the circle of animals that are tortoises

Thus, Bessie must be a vegetarian

Suppose every place in the world that people live is represented by the blue space inside the rectangle. Suppose the long pink oval represents all the wooden houses in the world. And, suppose the green circle represents Canada. The most logical conclusion one can draw from the figure is:

A. all wooden houses are found in Canada

B. Everyone lives in a wooden house

C. Some Canadians live in wooden houses

D. No one lives in Canada

Suppose the following statements are all true: Person L is shorter

than person X Person Y is shorter

than person L Person M is shorter

than person Y What additional

piece of information would be required to conclude that “Person Y is shorter than Person J”?

A. Person L is taller than JB. Person X is taller than JC. Person J is taller than LD. Person J is taller than

ME. Person M is taller than

Y

Solution: Answer CSolution: Answer CM < Y < L < XM < Y < L < XSo, if J is taller than L,So, if J is taller than L,Y must be shorter than JY must be shorter than J

A mother wants to order one large pizza, with exactly 5 toppings for her three picky children. She can choose from 7 toppings; cheese, mushrooms, olives, ham, sausage, onions, and pineapple. Julie says there has

to be pineapple Mona says there

cannot be any olives Rex says that if there

is going to be sausage, then there has to be ham too.

Which combination of toppings should she select if she is to satisfy all three children’s combined demands?

A. pineapple, onions, cheese, mushrooms, sausage

B. cheese, sausage, ham, olives, pineapple

C. cheese, mushrooms, ham, onions, pineapple

D. sausage, mushrooms, onions, cheese, and ham.

cheese

mushroom

olive

ham sausage

onion

pineapple

Fifi Yes

Mona No

Rex then ham

if sausage

Note: the statement “if sausage, then ham” doesn’t imply “If ham then sausage.” The obverse doesn’t necessarily follow.

Deductive reasoning is either “valid” or “invalid.” A deductive argument can’t be “sort of” valid.

If the reasoning employed in an argument is valid and the argument’s premises are true, then the argument is said to be sound. valid reasoning

+ true premises = sound argument

Inductive reasoning enjoys a wide range of probability; it can be plausible, possible, reasonable, credible, etc.

the inferences drawn may be placed on a continuum ranging from cogent at one end to fallacious at the other.

fallacious cogent

Deductive reasoning is commonly found in the natural sciences or “hard” sciences, less so in everyday arguments

Occasionally, everyday arguments do involve deductive reasoning: Example: “Two or

more persons are required to drive in the diamond lane. You don’t have two or more persons. Therefore you may not drive in the diamond lane”

Inductive reasoning is found in the courtroom, the boardroom, the classroom, and throughout the media

Most, but not all everyday arguments are based on induction Examples: The

“reasonable person” standard in civil law, and the “beyond a reasonable doubt” standard in criminal law