2. DDeedduuccttiioonn:: reasoning from general premises, which are
known or presumed to be known, to more specific,
certain conclusions.
IInndduuccttiioonn:: the process of observing data, recognizing
patterns and making a conjecture of your observations.
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. DDeedduuccttiioonn::
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.
IInndduuccttiioonn::
commonly known as
“informal logic,” or
“everyday argument”
involves drawing
uncertain inferences,
based on probabilistic
reasoning.
the conclusions
reached are probable,
reasonable, plausible,
believable.
4. 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. A sample of fifty
motorists who were
stopped by the CHP at a
sobriety checkpoint on a
Saturday at midnight
revealed that one in
four drivers were either
uninsured, intoxicated,
or both. Thus, if you get
involved in an accident
on the freeway there is a
25% chance the other
motorist will be drunk
or uninsured.
The Law of the Sea
treaty states that any
vessel beyond a 12 mile
limit is in international
waters. The treaty also
states that any vessel in
international waters
cannot be legally
stopped or boarded.
Therefore, when the U.S.
Coast Guard intercepts
boats coming from Cuba
or Haiti more than 12
miles from the U.S.
coast, it is violating the
Law of the Sea.
6. 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.”
7. vegetarian animals tortoises
Bessie
All tortoises
fall in the
circle of
animals that
are
vegetarians
Thus, Bessie
must be a
vegetarian
Bessie falls into the circle
of animals that are
tortoises
8. Suppose every place
in the world that
people live is
represented by the
blue space inside the
rectangle. Suppose
the long pink oval
represents all the
wooden houses in the
world. And, suppose
the green circle
represents Canada.
The most logical
conclusion one can
draw from the figure
is:
A. all wooden houses
are found in Canada
B. Everyone lives in a
wooden house
C. Some Canadians live
in wooden houses
D. No one lives in
Canada
9. Suppose the
following statements
are all true:
Person L is shorter
than person X
Person Y is shorter
than person L
Person M is shorter
than person Y
What additional
piece of information
would be required to
conclude that
“Person Y is shorter
than Person J”?
A. Person L is taller than J
B. Person X is taller than J
C. Person J is taller than L
D. Person J is taller than
M
E. Person M is taller than
Y
SSoolluuttiioonn:: AAnnsswweerr CC
MM << YY << LL << XX
SSoo,, iiff JJ iiss ttaalllleerr tthhaann LL,,
YY mmuusstt bbee sshhoorrtteerr tthhaann JJ
10. A mother wants to
order one large pizza,
with exactly 5 toppings
for her three picky
children. She can
choose from 7 toppings;
cheese, mushrooms,
olives, ham, sausage,
onions, and pineapple.
Julie says there has to
be pineapple
Mona says there
cannot be any olives
Rex says that if there is
going to be sausage,
then there has to be
ham too.
Which combination of
toppings should she
select if she is to satisfy
all three children’s
combined demands?
A. pineapple, onions,
cheese, mushrooms,
sausage
B. cheese, sausage, ham,
olives, pineapple
C. cheese, mushrooms,
ham, onions, pineapple
D. sausage, mushrooms,
onions, cheese, and
ham.
11. chee
se
mushro
om
oliv
e
ham sausag
e
onio
n
pineapp
le
Fifi Yes
Mona No
Rex then
ham
if
sausage
Note: the statement “if sausage, then ham” doesn’t
imply “If ham then sausage.” The obverse doesn’t
necessarily follow.
12. 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
13. 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
