Formal Versus Informal Logic

1. Formal Versus Informal Logic Deductive Versus Inductive Forms of Reasoning

2. Two basic categoriesof 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).

3. 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. Deduction Vs. Induction

4. 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 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 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 Deductive VersusInductive Reasoning

5. 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 Deductive and Inductive Arguments

6. 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

7. 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---continued fallacious cogent

8. 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 Deduction Versus Induction--still more