Deduction

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.
	Induction: reasoning from specific cases to more general, but uncertain, conclusions.
	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 Vs. Induction
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

commonly known as "informal logic," or "everyday argument"

involves drawing uncertain inferences, based on probabalistic reasoning.

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

Deductive Versus Inductive Reasoning
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

Sample Deductive and Inductive Arguments
Example of Deduction

major premise: All bovines have split hooves

minor premise: Bessie, my cow, is a bovine

conclusion: Therefore, Bessie has split hooves

Example of Induction

Boss to employee: "You need a new alarm clock, you’ve been late to work the last three mornings."

Deduction Versus Induction---continued

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.

Deduction Versus Induction--still more

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: "You must have 124 units to graduate. You don’t have 124 units, so you can’t graduate."

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

	Deductive reasoning is commonly found in the natural sciences or "hard" sciences, less so in everyday arguments
	Occasionally, everyday arguments do involve deductive reasoning:

	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