Logic and Reasoning
The philosopher’s toolbox
Why does logic matter?
 As a class, come up with some ways in which we use
logic every day. (5-10 minute brainstorm)
How do we use logic
 Make decisions
 Win arguments
 Assess truth
 Explore the dangers of dihydrogen monoxide at
www.dhmo.org
The death of a Canadian Icon
Logic - definition
 From Greek word logos (speech or reason)
 Logike tekhne is __________________________
 Area of philosophy that studies correct __________ and
sound judgement
 Helps to
__________________________________________________________
_____________. Exposes faulty arguments by examining
assumptions about what is true.
 Not concerned with ___________ of views (conservative vs.
liberal, generous vs. stingy). Only concerned with what can
and cannot be justified.
Deduction vs. Induction
General
Particular
Deduction
 _______________________________________________________
_______________________________________________________
All cats are
mammals
Spot is a cat
Spot is a
mammal
Induction
Based on observing ________________ things and making
_________________ (small picture to big picture)
- Graham leaves for
school at 7 am
- Graham is never late
Evidence
Conclusion
Three laws of thought
 Aristotle Organon
 Foundations of formal logic and deductive reasoning
a) ______________________________
b) ______________________________
c) ______________________________
Law of Non contradiction
 Something cannot be said to both be and not to be at
the same time.
 Example: If we conclude that the apple is the colour red,
it cannot be said to be ‘not red’ at the same time
Law of the Excluded middle
 Something must either be or not be.
 Example: Either Nikita exists or does not exist. There is no
third possibility or middle
Law of Identity
 Something is what it says it is
 Example: Carolina is Carolina, Bhavik is Bhavik. Bhavik
cannot be Carolina
Ship of Theseus
Beyond Aristotle
 12th century French monk Peter Abelard compiled 158
contradictory viewpoints on philosophical/theological
questions in Sic et Non
 17th century, Francis Bacon and scientific method sees
shift to inductive reasoning
 19th century mathematician George Boole uses chains of
deductiive reasoning in algebra
 Kurt Godel, mathematician proved that some
mathematical statements cannot be proven even if
correct rules and principles are applied
Terms
 Argument – ____________________ consisting of premise(s)
and a conclusion
 Premise – factual statement or ________________. Provide
reasons for believing a conclusion
 Syllogism – basic form of argument that uses
_______________ reasoning. A syllogism consists of at least
two premises and a conclusion
Syllogism
 All humans are mortal
 Aristotle is a human
 Therefore Aristotle is mortal
Write your own syllogism

Use any of the following words to
create a syllogism of your own

Apples

Baking

Cars

Dancehall reggae

Entropy

Fallacy

Gophers

Helicopter

Ice cream
Truth and Validity
 Truth- refers to the truth of
content within a statement
or premise
 Validity – refers to the
correctness of reasoning
 Logicians are more
concerned with validity
than the truth of an
argument’s statements
Valid but untrue
 Premise: all humans are immortal
 Premise: I am human
 Conclusion: Therefore I am immortal
Are these true? Are these valid?
Premise is untrue but arguments are still valid because the
conclusions follows logically from the premises
Validity, Soundness and Reliability
 In deductive arguments, the truth of the premises
guarantees the truth of the conclusion. An argument is
said to be sound if its form is valid and its premises are
true.
 The same test cannot be applied to inductive arguments
which are identified as “reliable” or “unreliable”