Formal Versus Informal Logic. Deductive Versus Inductive Forms of Reasoning. Two basic categories of human reasoning. Deduction: reasoning from general premises, which are known or presumed to be known, to more specific, certain conclusions.
Deductive Versus Inductive
Forms of Reasoning
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.
commonly known as “informal logic,” or “everyday argument”
involves drawing uncertain inferences, based on probabalistic reasoning.
the conclusions reached are probable, reasonable, plausible, believable.Deduction Vs. Induction
It is the form or structure of a deductive argument that determines its validity
the fundamental property of a valid, deductive argument is that ifthe premises are true, thenthe conclusion necessarily follows.
The conclusion is said to be “entailed” in, or contained in, the premises.
example: use of DNA testing to establish paternity
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 argumentDeductive VersusInductive Reasoning
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.Inductive or deductive reasoning?
Thus, Bessie must be a vegetarian
All tortoises fall in the circle of animals that are vegetarians
Bessie falls into the circle of animals that are tortoises
all wooden houses are found in Canada
Everyone lives in a wooden house
Some Canadians live in wooden houses
No one lives in CanadaOther types ofdeductive arguments
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”?
Person L is taller than J
Person X is taller than J
Person J is taller than L
Person J is taller than M
Person M is taller than YOther types ofdeductive arguments
Solution: Answer C
M < Y < L < X
So, if J is taller than L,
Y must be shorter than J
Fifi 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?
pineapple, onions, cheese, mushrooms, sausage
cheese, sausage, ham, olives, pineapple
cheese, mushrooms, ham, onions, pineapple
sausage, mushrooms, onions, cheese, and ham.Other types ofdeductive arguments
Note: the statement “if sausage, then ham” doesn’t imply “If ham then sausage.” The obverse doesn’t necessarily follow.
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.Deduction Versus Induction---continued
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 lawDeduction Versus Induction--still more