Deductive reasoning
By-Pushpendra singh
“Deductive reasoning, or deduction, starts with a general case and deduces specific instances.
Deduction starts with an assumed hypothesis or theory, which is why it has been called
'hypothetico-deduction'. This assumption may be well-accepted or it may be rather shakier -nevertheless, for the argument it is not questioned.
Deduction is used by scientists who take a general scientific law and apply it to a certain case, as
they assume that the law is true. Deduction can also be used to test an induction by applying it
elsewhere, although in this case the initial theory is assumed to be true only temporarily.”
( http://changingminds.org/disciplines/argument/types_reasoning/deduction.htm)
It is a type of reasoning which goes from general to specific. Deductive reasoning is based on
premises and if the premises are true, then the reasoning will be valid.
Deductive Reasoning: Everyday Examples
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All apples are fruits, all fruits grow on trees; therefore, all apples grow on trees.
William is a bachelor, all bachelors are single; hence William is single.
Since all humans are mortal, and I am a human, then I am mortal.
All dolphins are mammals, all mammals have kidneys; therefore all dolphins have
kidneys.
Since all squares are rectangles, and all rectangles have four sides, so all squares
have four sides.
If Dennis misses work and at work there is a party, then Dennis will miss the party.
All numbers ending in 0 or 5 are divisible by 5. The number 35 ends with a 5, so it is
divisible by 5.
The Earth is a planet, and all planets orbit a sun, therefore the Earth orbits a sun.
To earn a master’s degree, a student must have 32 credits. Tim has 40 credits, so
Tim will earn a master’s degree.
All birds have feathers and robins are birds, so robins have feathers.
It is dangerous to drive on icy streets. The streets are icy now so it is dangerous to
drive now.
All cats have a keen sense of smell. Fluffy is a cat, so Fluffy has a keen sense of
smell.
The elm is a tree and all trees have bark, so elms have bark.
Snakes are reptiles and reptiles are cold-blooded; therefore, snakes are coldblooded.
Cacti are plants and all plants perform photosynthesis; therefore, cacti perform
photosynthesis.
Red meat has iron in it and beef is red meat, so beef has iron in it.
Acute angles are less than 90 degrees and this angle is 40 degrees so this angle is
acute.
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All noble gases are stable and helium is a noble gas, so helium is stable.
Magnolias are dicots and dicots have two embryonic leaves; therefore magnolias
have two embryonic leaves.
Elephants have cells in their bodies and all cells have DNA, so elephants have
DNA.
All cars have at least two doors and a Ford Focus is a car, so the Ford Focus has at
least two doors.
All horses have manes and the Arabian is a horse; therefore Arabians have manes.
(http://examples.yourdictionary.com/deductive-reasoning-examples.html)
Deductive reasoning is a basic form of valid reasoning. Deductive reasoning, or deduction, starts
out with a general statement, or hypothesis, and examines the possibilities to reach a specific,
logical conclusion. The scientific method uses deduction to test hypotheses and theories.
In deductive reasoning, if something is true of a class of things in general, it is also true for all
members of that class. For example, "All men are mortal. Harold is a man. Therefore, Harold is
mortal." For deductive reasoning to be sound, the hypothesis must be correct. It is assumed that
the premises, "All men are mortal" and "Harold is a man" are true. Therefore, the conclusion is
logical and true.
It's possible to come to a logical conclusion even if the generalization is not true. If the
generalization is wrong, the conclusion may be logical, but it may also be untrue. For example,
the argument, "All bald men are grandfathers. Harold is bald. Therefore, Harold is a
grandfather," is valid logically but it is untrue because the original statement is false.
Inductive reasoning
Inductive reasoning is the opposite of deductive reasoning. Inductive reasoning makes broad
generalizations from specific observations. Even if all of the premises are true in a statement,
inductive reasoning allows for the conclusion to be false. Here’s an example: "Harold is a
grandfather. Harold is bald. Therefore, all grandfathers are bald." The conclusion does not follow
logically from the statements.
(http://www.livescience.com/21569-deduction-vs-induction.html)
"Deduction" as reasoning from the general to specific and define "induction" as reasoning from
the specific to the general. These definitions are outdated and inaccurate. For example, according
to the more modern definitions given above, the following argument from the specific to general
is deductive, not inductive, because the truth of the premises guarantees the truth of the
conclusion:
The members of the Williams family are Susan, Nathan and Alexander.
Susan wears glasses.
Nathan wears glasses.
Alexander wears glasses.
Therefore, all members of the Williams family wear glasses.
(http://www.iep.utm.edu/ded-ind/)