Induction vs. deduction in science

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Induction vs. deduction in science
(Photo: Bethany Carlson, stock.xchng (

Science is a discipline that is based in and relies on logic. There are two general classes of logical reasoning: Inductive and deductive. The type of scientific logic that most people think of is deductive. However, inductive reasoning plays an important part in science and scientific research.

Overview of Deductive Reasoning

Deductive reasoning uses a top-down approach to logic. Deductive reasoning usually starts with a theory. A hypothesis, or a definition of a theory suitable for testing, follows. Observations from the tests of the hypothesis either confirm or disprove the hypothesis and theory. Deductive reasoning is often called Aristotellian logic or inference.

Examples of Deductive Reasoning

Deductive reasoning is easily explained using symbolic logic. For example, if A is equal to B and B is equal to C, then A is equal to C. A second example of deductive reasoning is as follows:

  1. All people are mortal.
  2. Aristotle is a man.

Therefore: Aristotle is mortal.

An example of deductive reasoning that doesn't work is:

  1. All criminals oppose government.
  2. Everyone in the opposition opposes the government.

Therefore: Everyone in the opposition is a criminal.

This example shows the fallacy of the undistributed middle, because there is no logical link between being in the opposition and being a criminal. In addition, the initial step in the argument, that all criminals oppose the government, falls apart under closer examination.

Overview of Inductive Reasoning

Inductive reasoning is a bottom-up method of logic. By beginning with an observation or several observations, a pattern can be recognised. Once the pattern is defined, it becomes a tentative hypothesis. Once the hypothesis is defined and tested, the hypothesis and theory can be proven or disproved.

Example of Inductive Reasoning

Inductive reasoning can be more complex, but can also be more simple. An example of induction is as follows:

All crows that I've seen are black, therefore all crows are black.

Charles Darwin used inductive reasoning in his development of his theories on evolution. He observed differences in finches on different islands in the Galapagos Islands. After more observation, he noticed that the finches were geographically isolated. He reasoned that because of their similarities, all finches on the islands came from a common ancestor, but had evolved and adapted to the unique requirements of each island.

When to Use the Two Methods

Inductive reasoning and deductive reasoning can work well together in science. Initial theories developed using induction can then be tested using deduction. In some cases where the theory is very complex, induction can result in a valid hypothesis in less time than if deduction were used.

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