Logic: Deduction and Induction

1. Two Types of Arguments
Session 3

2. Basics Types of argument; deductive &
inductive
• Deductive Argument:
• In a deductive argument the conclusion follows from the premise(s)
with necessity so that, if all of the premises are true, then the
conclusion cannot be false
All men are mortal
Socrates is a man
Therefore Socrates is mortal

3. Basics Types of argument; deductive &
inductive
Inductive Argument:
In an Inductive argument the conclusion follows from the premise(s)
with some degree of probability. Thus even if premises are true there
are chances that the conclusion can be false
Most Greeks eat olives.
Socrates is a Greek.
Therefore, Socrates eats olives.

4. Inductive argument
• The conclusion in inductive argument remains acceptable
unless new evidences or facts come in observation.
• The conclusion that “Therefore, Socrates eats olives” may be
proved to be false if we come to know that Socrates was
allergic to olives.

5. EVALUATING ARGUMENTS: Differentiating
between good and bad argument
• Deductive arguments are evaluated as valid or invalid and
sound or unsound
• Inductive arguments are evaluated as strong or weak

6. Valid Deductive argument:
• A valid deductive argument is an argument in which it is
impossible for the conclusion to be false given that the premises
are true. In these arguments the conclusion follows with strict
necessity from the premises.
• Example:
All vehicles need energy to move
Bicycle is a vehicle
Therefore, bicycle need energy to move
• * If all premises are true then the conclusion must be true

7. Invalid argument
• An invalid deductive argument is a deductive argument in which it is
possible for the conclusion to be false given that the premises are
true. In these arguments the conclusion does not follow with strict
necessity from the premises, even though it is claimed to.
• Example
• Anyone who lives in the city of Karachi, also lives in Pakistan.
• Suhail lives in Pakistan.
• Therefore, Suhail lives in the city of Karachi

8. Sound and unsound deductive argument
• If a deductive argument is valid and has all true premises, it is sound.
• Example
All mammals have lungs.
All whales are mammals.
Therefore all whales have lungs.
• If the argument is valid but has false premises, it is unsound.
• Example
All men are rich
All baggers are men
Therefore all baggers are rich

9. Inductive argument: strong & weak
• Inductive arguments are evaluated as either strong or weak and good
or bad, but never as valid and invalid
• An argument is generally strong if the probability that the conclusion
is true given the premises is greater than 50%.
• Cogent = Strong + All true premises
• Example:
• Almost all Republicans are conservative.
• John McCain is a Republican.
• Therefore, (it is likely that) McCain is conservative.

10. Can you evaluate the following inductive
argument?
• Every previous U.S. president was a TV debater.
Therefore, probably the next U.S. president will be a TV debater.
Weak? Strong? Cogent? Uncogent?

11. Thank you