There are two main categories of reasoning: deductive and inductive. Deductive reasoning involves drawing certain conclusions from known or presumed general premises, while inductive reasoning involves drawing general but uncertain conclusions from specific cases. Both forms can be compelling, with deduction relying on logically valid arguments and induction relying on probabilistic reasoning deemed plausible by the receiver. Common examples provided include determining vegetarianism from tortoise classification (deduction) and inferring Navy service from a tattoo (induction).

1. Formal Versus Informal Logic
Deductive Versus Inductive
Forms of Reasoning

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