2. Two basic categories
of 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 Vs. Induction
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.
4. Deductive Versus
Inductive Reasoning
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 if the
premises are true, then
the 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
5. Sample Deductive and Inductive Arguments
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.”
6. Bessie
tortoises
vegetarian animals
sample “Venn diagram”
of a deductive argument
All tortoises
fall in the
circle of
animals that
are
vegetarians
Bessie falls into the circle
of animals that are
tortoises
Thus, Bessie
must be a
vegetarian
7. Deduction Versus Induction
---continued
• 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.
fallacious cogent
8. Deduction Versus Induction
--still more
• 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
