Formal Versus Informal Logic Deductive Versus Inductive Forms

  • Slides: 8
Download presentation
Formal Versus Informal Logic Deductive Versus Inductive Forms of Reasoning

Formal Versus Informal Logic Deductive Versus Inductive Forms of Reasoning

Two basic categories of human reasoning • Deduction: reasoning from general premises, which are

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: Induction • commonly associated • with “formal logic. ” •

Deduction Vs. Induction Deduction: Induction • 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.

Deductive Versus Inductive Reasoning Deduction • It is the form or structure of a

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 3 rd party) determines the worth of an inductive argument

Sample Deductive and Inductive Arguments Example of Deduction • major premise: All tortoises are

Sample Deductive and Inductive Arguments 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. ”

sample “Venn diagram” of a deductive argument vegetarian animals tortoises Thus, Bessie must be

sample “Venn diagram” of a deductive argument vegetarian animals tortoises 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 Bessie

Deduction Versus Induction ---continued • Deductive reasoning is either “valid” or “invalid. ” A

Deduction Versus Induction ---continued • Deductive reasoning is either “valid” or “invalid. ” A deductive argument can’t be “sort of” valid. • 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. • 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 fallacious + true premises = sound argument cogent

Deduction Versus Induction --still more • Deductive reasoning is commonly found in the natural

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: “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