Deductive reasoning

Deductive reasoning, according to many dictionaries, is the type of reasoning that proceeds from general principles or premises to derive particular information.

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. Many incorrectly teach that deductive reasoning goes from general information to specific information and that inductive reasoning travels in the opposite direction. This is not accurate. Deductive reasoning applies general principles to reach specific conclusions, whereas inductive reasoning examines specific information, perhaps many pieces of specific information, to derive 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).

Both types of reasoning are routinely employed. One difference between them is that in deductive reasoning, the evidence provided must be a set about which everything is known before the conclusion can be drawn. Since it is difficult to know everything before drawing a conclusion, deductive reasoning has little use in the real world. This is where inductive reasoning steps in. Given a set of evidence, however incomplete the knowledge is, the conclusion is likely to follow, but one gives up the guarantee that the conclusion follows. However it does provide the ability to learn new things that are not obvious from the evidence.

Deductive logic

Deductive reasoning is supported by deductive logic (which is not 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 fruit is red.

Intuitively, one might deny the major premise or 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 reading

• Vincent F. Hendricks, Thought 2 Talk: A Crash Course in Reflection and Expression, New York: Automatic Press / VIP, 2005, ISBN 87-991013-7-8

• Zarefsky, David, Argumentation: The Study of Effective Reasoning Parts I and II, The Teaching Company 2002

See also

• Abductive reasoning
• Analogical reasoning
• Correspondence theory of truth
• Defeasible reasoning
• Hypothetico-deductive method
• Inquiry

• Logical consequence
• Propositional calculus
• Retroductive reasoning
• Scientific method
• Soundness
• Validity