Formal Versus Informal Logic

Deductive Versus Inductive Forms of Reasoning

Two basic categories of human reasoning

bulletDeduction: reasoning from general premises, which are known or presumed to be known, to more specific, certain conclusions.
bulletInduction: reasoning from specific cases to more general, but uncertain, conclusions.
bulletBoth 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


bulletcommonly associated with "formal logic."
bulletinvolves reasoning from known premises, or premises presumed to be true, to a certain conclusion.
bulletthe conclusions reached are certain, inevitable, inescapable.



bulletcommonly known as "informal logic," or "everyday argument"
bulletinvolves drawing uncertain inferences, based on probabalistic reasoning.
bulletthe conclusions reached are probable, reasonable, plausible, believable.

Deductive Versus Inductive Reasoning


bulletIt is the form or structure of a deductive argument that determines its validity
bulletthe fundamental property of a valid, deductive argument is that if the premises are true, then the conclusion necessarily follows.
bulletThe conclusion is said to be "entailed" in, or contained in, the premises.
bulletexample: use of DNA testing to establish paternity



bulletBy 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.
bulletThe receiver (or a 3rd party) determines the worth of an inductive argument

Sample Deductive and Inductive Arguments

Example of Deduction

bulletmajor premise: All bovines have split hooves
bulletminor premise: Bessie, my cow, is a bovine
bulletconclusion: Therefore, Bessie has split hooves

Example of Induction

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

Deduction Versus Induction---continued

bulletDeductive reasoning is either "valid" or "invalid." A deductive argument can’t be "sort of" valid.
bulletIf 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

bulletInductive reasoning enjoys a wide range of probability; it can be plausible, possible, reasonable, credible, etc.
bulletthe inferences drawn may be placed on a continuum ranging from cogent at one end to fallacious at the other.

Deduction Versus Induction--still more

bulletDeductive reasoning is commonly found in the natural sciences or "hard" sciences, less so in everyday arguments
bulletOccasionally, 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."

bulletInductive reasoning is found in the courtroom, the boardroom, the classroom, and throughout the media
bulletMost, but not all everyday arguments are based on induction
bulletExamples: The "reasonable person" standard in civil law, and the "beyond a reasonable doubt" standard in criminal law