Philosophy: Logic and Logical arguments 8.2 Forensics October 30, 2014

Objectives: SWBAT Identify the basic forms of logical argument Identify the basic pieces of a logical argument Construct logical arguments

Introduction to Logic Logic The science of correctness or incorrectness of reasoning, the study of the evaluation of arguments A STATEMENT is a declarative sentence, or a part of a sentence Either true or false The Winter Olympics are in Russia this year, but four years ago they were in Vancouver, Canada. A PROPOSITION is what is meant by the statement The idea it expresses

Basic Concepts An ARGUMENT PREMISES CONCLUSION Collection of statements or propositions, some of which are intended to provide support or evidence for others PREMISES Statements or propositions in an argument that are intended to provide support or evidence CONCLUSION Statement or proposition for which the premises provide support

Example arguments P1: If the President lies to Congress, then he should be impeached P2: The President lied to Congress C: Therefore, he should be impeached P1: If everything in the Bible is true, then the world was created in six days P2: The world was not created in six days C: Therefore, not everything in the Bible is true.

What makes a good argument? DEDUCTIVELY VALID Impossible for the conclusion to be false if the premises are true FACTUALLY CORRECT The premises are true A SOUND argument is one that is Deductively valid AND Factually correct

Modus Ponens Premises Conclusion Aristotle called this modus ponens: (a rule of inference – one of the most important rules) if you study then you succeed you study you succeed Premises Conclusion Aristotle called this modus ponens: if P then Q P Q Discussion #8

Argument Form Multiple modus ponens (MMP) Modus tollens (MT) Whether or not an argument is valid depends on its form Other valid forms Multiple modus ponens (MMP) if P then Q If Q the R P Therefore, R Modus tollens (MT) if P then Q Not Q Therefore, not P

Example arguments P1: If the President lies to Congress, then he should be impeached P2: he President lied to Congress C: Therefore, he should be impeached P1: If everything in the Bible is true, then the world was created in six days P2: The world was not created in six days C: Therefore, not everything in the Bible is true.

Common Valid Forms Hypothetical Syllogism (HS) if P then Q If Q then R Therefore, if P then R Disjunctive Syllogism (DS) Either P or Q Not P Therefore, Q Constructive Dilemma (CD) Either P or Q If P then R If Q then R Therefore, R

Invalid Forms If P then Q If P then Q Q Not P Therefore, P Therefore, not Q If Hillary Clinton is a Communist Spy, then she supports Obamacare She supports Obamacare Therefore, she is a communist spy If JFK jumped off the Eiffel Tower, Then he is dead He did not jump off the Eiffel Tower Therefore he is not dead

Evaluating Arguments Does the argument have a valid form? Are the premises true? If the answer is yes to both, then the argument is sound If the argument is sound then the conclusion is true

Philosophy: Evaluating Logical arguments 8.2 Forensics December 4, 2013

Objective Evaluating arguments and the use of logic in their construction

Evaluating Arguments Logical evaluation Does the argument have a valid form Are the premises true? If the answer to both questions is “yes,” then the argument is sound and its conclusion is true.

Evaluating Arguments If you think the conclusion is false The form must be invalid The premise must be false All acts of killing humans are morally wrong If all acts of killing humans are morally wrong then abortion is morally wrong Therefore abortion is always morally wrong

Evaluating Arguments You can’t get much out of an unsound argument An unsound argument can still have a true conclusion All hamsters are refrigerators All refrigerators are mammals All hamsters are mammals

Be clear – is the ARGUMENT faulty, or is the CONCLUSION false? Evaluating Arguments If God exists, then God created everything in the Universe If God created everything in the Universe then everything in the Universe is good If everything in the universe is good, then unnecessary pain and suffering does not exist Unnecessary pain and suffering does exist Therefore, God does not exist Be clear – is the ARGUMENT faulty, or is the CONCLUSION false?

The Purpose of Logical Rigor Why so serious? Most of the issues we will be discussing will be controversial We need to figure out WHY we disagree when we do Many of us have attitudes about ethics that are incongruous with one another

The Purpose of Logical Rigor All acts of killing humans is morally wrong If all acts of killing humans are morally wrong, then abortion is always morally wrong Therefore, abortion is always morally wrong But what about the death penalty?

The Purpose of Logical Rigor Refining the argument All acts of killing innocent humans are morally wrong If all acts of killing innocent…. Therefore, abortion is always morally wrong