What Is the Difference Between Inductive and Deductive Logic?

Deduction logic uses a hypothesis and examines the possibilities in order to reach a logical conclusion and inductive reasoning makes wide sweeping generalizations from a set of specific observations. The scientific method requires deductive logic to test a hypothesis and scientists also use inductive reasoning to form hypotheses.

Deductive logic, also known as the "from the top down" approach, starts with a general idea and works down to the details. A hypothesis is generally formed using a syllogism, or a three-stepped argument. For example:

  1. Every plant is carbon-based
  2. This thing in my hand is a plant
  3. Therefore, this plant is carbon-based
If the plant was not carbon-based, further experiments would be required.

Because of the broad generalizations used in inductive logic, there is a reasonable chance that this leap-of-faith assumption is wrong. An example of inductive logic is "Elizabeth is a woman and has long hair, therefore all women have long hair." While the conclusion is false, it allows scientists to form theories and hypotheses to test.

Deduction can give absolute proof to a theory as long as the premises are correct, however if the premises remain unproven, they must be accepted at face value. Inductive logic may be driven by observations and measurements, it never offers proof of a theory. The scientific method uses a mixture of deductive and inductive reasoning to work.