(4) Deduction versus Induction

Deduction and induction are the two broad methods of reason in logic.  Deductive reasoning takes a “top-down” approach whereas inductive reasoning take a “bottom-up” approach.  Simply put, deduction is focused with confirming a theory whereas induction is focused with how a theory is developed.

Deductive reasoning starts with accepted theory (general explanation) and narrows down to specific observations of truth.  This method of reasoning is often used to confirm or contradict the underlying theory.  Deductive reasoning follows accepted truths (premises) and infers from those a necessary conclusion.  If the premises are true and rules of logic are followed, the conclusion holds true.  If the conclusion turns out to be false, one or more of the premises must be false.

Inductive reasoning starts with observing specific patterns and then developing a theory (general explanation) to explain the observations.  Because there are no accepted truths to build upon with inductive reasoning, arguments are considered strong or weak as opposed to valid or invalid and conclusions are not stated as necessary and certain but rather as probable.   The strength of the inductive argument centers around the value of provided evidence.

When seeking out what is true, it is best to employ both methods of reasoning.  This has been demonstrated over time with the advancement of scientific understanding.  An individual employs inductive reasoning to create a theory to explain observations or patterns.  Other individuals then work to disprove parts of the theory or its entirety using deductive reasoning.  Theories become accepted as truth if they are strong enough to withstand years of attempts at disproving its tenets.  It is noteworthy to recognize that accepting that a theory as true does not necessarily imply that the theory is true but rather that it is highly probable that the theory is true.

The following shows an example that demonstrates inductive reasoning versus deductive reasoning.

  1. Inductive Process
    1. An observation is made that all crows seem to be black.
    2. An attempt to is made to determine a pattern by observing the color of crows at multiple locations in the world.
    3. A generalization is made that ALL crows are black.  The argument can only strongly infer that all crows are black.  It does not and cannot prove this with certainty because all crows were not observed to confirm the black color.
  2. Deductive Process
    1. To disprove the generalization that ALL crows are black one only need find a single non-black crow.
    2. The theory (generalization) that ALL crows are black will continue to gain acceptance as long as continued searches fail to discover a single non-black crow.

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