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Deductive Versus Inductive Reasoning
http://www.nakedscience.org/mrg/Deductive%20and%20Induct...
DEDUCTIVE VERSUS INDUCTIVE REASONING
Inductive and deductive reasoning are two methods of logic used to arrive at a conclusion based on
information assumed to be true. Both are used in research to establish hypotheses.
Deductive reasoning arrives at a specific conclusion based on generalizations. Inductive reasoning
takes events and makes generalizations
Deductive reasoning is reasoning that involves a hierarchy of statements or truths. Starting with a
limited number of simple statements or assumptions, more complex statements can be built up from
the more basic ones. For example, you have probably studied deductive geometry in mathematics; in
it you start with a few principles and prove various propositions using those principles. To prove more
complicated propositions, you may use propositions that you have already proved plus the original
principles. In more formal logic terms deductive reasoning is reasoning from stated premises to
conclusions formally or necessarily implied by such premises.
Deductive reasoning can be described as reasoning of the form if A then B. Deduction is in some
sense the direct application of knowledge in the production of new knowledge.
If-then deductive reasoning is how scientists (and other people!) can test alternate hypotheses. Making
deductions is important when we cannot directly observe a cause, and can only observe its
consequences. This kind of reasoning can be modeled by the following:
If ...
Then...
But...
Therefore...
For example, we might hypothesize that "The color of a mineral is determined by its crystal
structure."
And so we could test this hypothesis using deductive reasoning:
If the color of a mineral is determined by its crystal structure; then all purple minerals should have the
same crystal structure. But purple amethyst has a hexagonal structure and purple fluorite has an
isometric structure (determined by observations). Therefore, the hypothesis is not supported or
strengthened.
Inductive reasoning is essentially the opposite of deductive reasoning. It involves trying to create
general principles by starting with many specific instances. For example, in inductive geometry you
might measure the interior angles of a group of randomly drawn triangles. When you discover that the
sum of the three angles is 180° regardless of the triangle, you would be tempted to make a
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Deductive Versus Inductive Reasoning
http://www.nakedscience.org/mrg/Deductive%20and%20Induct...
generalization about the sum of the interior angles of a triangle. Bringing forward all these separate
facts provides evidence in order to help support your general statement about the interior angles.
This is the kind of reasoning used if you have gradually built up an understanding of how something
works. Rather than starting with laws and principles and making deductions, most people collect
relevant experience and try to construct principles from it.
Again the distinction between the two types of reasoning is not always sharp. In mathematics it is
important to know which kind of formal system you are using and to stick to it. Inductive proofs are
not allowed in a deductive system.
Inductive reasoning progresses from observations of individual cases to the development of a
generality. (Inductive reasoning, or induction, is often confused with deductive thinking; in the latter,
general principles or conditions are applied to specific instances or situations.) If a child puts his or
her hand into a bag of candy and withdraws three pieces, all of which are red, he or she may conclude
that all the candy is red. Inductive reasoning, or induction, is the process by which a general
conclusion is reached from evaluating specific observations or situations.
Many people distinguish between two basic kinds of argument: inductive and deductive. Induction is
usually described as moving from the specific to the general, while deduction begins with the general
and ends with the specific; arguments based on experience or observation are best expressed
inductively, while arguments based on laws, rules, or other widely accepted principles are best
expressed deductively.
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