Everything about Deductive Reasoning totally explained
Deductive reasoning is
reasoning which uses deductive
arguments to move from given statements (
premises), which are assumed to be true, to
conclusions, which must be true if the premises are true.
The classic example of deductive reasoning, given by
Aristotle, is
- All men are mortal. (major premise)
- Socrates is a man. (minor premise)
- Socrates is mortal. (conclusion)
Deductive reasoning is often contrasted with
inductive reasoning, which reasons from a large number of particular examples to a general rule.
Background
Deductive reasoning was developed by
Aristotle,
Thales,
Pythagoras, and other Greek philosophers of the Classical Period (600 to 300 B.C.). Aristotle, for example, relates a story of how Thales used his skills to deduce that the next season's olive crop would be a very large one. He therefore bought all the olive presses and made a fortune when the bumper olive crop did indeed arrive.
Deductive reasoning is dependent on its premises. That is, a false premise can possibly lead to a false result, and inconclusive premises will also yield an inconclusive conclusion.
Alternative to deductive reasoning is
inductive reasoning. The basic difference between the two can be summarized in
the deductive dynamic of logically progressing from general evidence to a particular truth or conclusion; whereas with induction the logical dynamic is precisely the reverse. Inductive reasoning starts with a particular observation that's believed to be a demonstrative model for a truth or principle that's assumed to apply generally.
Deductive reasoning applies general
principles to reach specific conclusions, whereas inductive reasoning examines specific information, perhaps many pieces of specific information, to impute a general principle. By thinking about phenomena such as how apples fall and how the planets move,
Isaac Newton induced his
theory of gravity. In the
19th century,
Adams and LeVerrier applied Newton's theory (general principle) to deduce the existence,
mass, position, and
orbit of
Neptune (specific conclusions) from perturbations in the observed orbit of
Uranus (specific data).
Deductive logic
Deductive reasoning is supported by deductive logic (which isn't quite the same thing).
For example:
» All apples are fruit.
All fruits grow on trees.
» Therefore all apples grow on trees.
Or
» All apples are fruit.
Some apples are red.
» Therefore some fruits are red.
Intuitively, one might deny the major premise and hence the conclusion; yet anyone accepting the premises accepts the conclusion.
Natural deduction
Deductive reasoning should be distinguished from the related concept of
natural deduction, an approach to proof theory that attempts to provide a formal model of logical reasoning as it "naturally" occurs.
Cultural references
Sherlock Holmes, the
fictional detective created by
Sir Arthur Conan Doyle, is well known for referring to deductive reasoning in numerous of Doyle's stories.
Further Information
Get more info on 'Deductive Reasoning'.
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