What is deductive reasoning?

Deductive reasoning, also known as deduction, is a basic form of reasoning that uses a general principle or premise as grounds to draw specific conclusions. 

This type of reasoning leads to valid conclusions when the premise is known to be true — for example, "all spiders have eight legs" is known to be a true statement. Based on that premise, one can reasonably conclude that, because tarantulas are spiders, they, too, must have eight legs.

The scientific method uses deduction to test scientific hypotheses and theories, which predict certain outcomes if they are correct, said Sylvia Wassertheil-Smoller, a researcher and professor emerita at Albert Einstein College of Medicine. 

"We go from the general — the theory — to the specific — the observations," Wassertheil-Smoller told Live Science. In other words, theories and hypotheses can be built on past knowledge and accepted rules, and then tests are conducted to see whether those known principles apply to a specific case.

Deductive reasoning begins with a first premise, which is followed by a second premise and an inference, or a conclusion based on reasoning and evidence. A common form of deductive reasoning is the "syllogism," in which two statements — a major premise and a minor premise — together reach a logical conclusion. 

For example, the major premise "Every A is B" could be followed by the minor premise "This C is A." Those statements would lead to the conclusion that "This C is B." Syllogisms are considered a good way to test deductive reasoning to make sure the argument is valid.

In deductive reasoning, if something is true of a class of things in general, it is also true for all members of that class. 

Deductive conclusions are reliable provided that the premises they're based on are true, but you run into trouble if they're false, according to Norman Herr, a professor of secondary education at California State University, Northridge. For instance the argument "All bald men are grandfathers. Harold is bald. Therefore, Harold is a grandfather," is logically valid, but it is untrue because the original premise is false.

Related: Crows outthink monkeys, can grasp recursive patterns 

Deductive reasoning examples

Here are some examples of deductive reasoning:

Major premise: All mammals have backbones.
Minor premise: Humans are mammals.
Conclusion: Humans have backbones.

Major premise: All birds lay eggs.
Minor premise: Pigeons are birds.
Conclusion: Pigeons lay eggs.

Major premise: All plants perform photosynthesis.
Minor premise: A cactus is a plant.
Conclusion: A cactus performs photosynthesis.