Transcript Logic and Reasoning

Logic and Reasoning
Panther Prep
North Central High School
Logical Proof
Logical reasoning is the easiest way to
convince because the information is
presented as fact.
However, not all information is common
sense to the audience so the speaker has to
connect the dots.
To do this, the speaker can use:
– Deductive reasoning
– Inductive reasoning
Introduction
Logic
– the study of reasoning
– allows us to test the
truth or falsity of
inferences and
reasoning
– used in problem
solving
Inference
…reaching a conclusion on the basis of
evidence and reasoning
– the creation of new ideas or information from
existing ideas or information
– the process of creating new statements from other
statements
• these statements are called Propositions
• Propositional Logic—the logic of propositions—analyzing
the validity of propositions
Induction
Inductive reasoning
– the inference of a general law from particular
instances
– the process of summarizing observations to derive
a generalization about the world
– we rarely observe all of the instances if a
particular phenomena, so we can never be sure it
is true in all cases—we can estimate probability
Induction
 A daisy is a flower.
 A rose is a flower.
 A violet is a flower.
Therefore, plants with colorful blooms are
flowers.
Deduction
Deductive reasoning is a way for the speaker to draw
a direct line from a general concept to the specific
end of their purpose.
…the inference of particular instances by reference to
a general law or principle
Sherlock Holmes’ deductive reasoning techniques
The format for these sets is called a syllogism
Syllogisms



All men are mortal.
Socrates is a man.
Therefore, Socrates is mortal.
–
The 1st statement is the generalization or general rule called the
major premise
–
The 2nd statement is an observation or specific fact called the
minor premise
–
The 3rd statement is the inference that follows the first two
statements called the conclusion
Validity and Truth
The Rules of Inference tell us whether a
syllogism is valid or not, but not if it is true.
– Validity is simply a matter of correct reasoning.
To determine if a conclusion of a valid
syllogism is true, we have to agree that the
premises are true.
– When a syllogism is both valid in reasoning and
the premises are true, we say the conclusion is
sound.
Accuracy vs. Precision
Accuracy: proximity
to target or true
value
Accurate,
but not
precise
Precision: proximity
to similar situations
Precise,
but not
accurate
Fallacies
Fallacies are mistruths that are used to make
a point.
Fallacies are generally used to dramatize a
position, dramatize information or to
compensate for the lack of supporting
information.
While popularly used, they can lead to a
destruction of the credibility of the speaker
and therefore a destruction of the speech
itself.
Types of Fallacies
Types of fallacies include:
– Unwarranted or Hasty Generalization (Including information
that does not apply.)
– Errors in casual information
• Post Hoc – Unproven cause-effect relationship
• Non Sequitur – Stating a conclusion that does not necessarily
follow the argument.
– Begging the Question (States as fact what needs to be
proven.)
– Ignoring the Question (Misdirection of attitude)
Types of Fallacies (Cont.)
Types of fallacies include:
– False Analogy (A parallel that does not
exist.)
– Either/Or Fallacy (Stating only two options
when more exist.)
– Ad Hominem (Attack of character on the
opposition)
– Red Herring (Addition of information to
confuse)
– Ad Populum (Public opinion MUST be
right, right?)
Identifying Fallacies
See if you can determine what, if any,
fallacy is present in the following
statements:
– “The Ford Explorer is the most popular SUV on the market, so it
must be the safest.”
– “You were the only person working when the accident happened
so it must be your fault.”
– “If you don’t finish this paper, you will become a bum and live in a
van down by the river.”
– “The rate of crime is increasing in our schools because of the
reduction of art and music programs.”
Identifying Fallacies (Cont.)
– “If you do not vote for the private school funding bill you
simply do not care about children.”
– “Stand back! I can help. My dad is a doctor.”
– “Johnny is not fit to lead this project because he can’t even
tie his shoes in the morning.”
– “Mom, if you don’t buy me an iPhone, it means that you
don’t really love me.”
– “Would we not work harder if we made more money?”
– “You either finish the project tonight or you will get eaten by
a werewolf.”
Validity and Truth
Validity does not guarantee truth and
truth does not guarantee validity.
In order to have a sound syllogism, the
form must be valid and the premises
must be true.