Deductive reasoning

method of reasoning by which premises understood to be true produce logically certain conclusions

Deduction is one of the two main types of reasoning. The other is induction. In deduction, we apply a general rule to a particular case.

Deductive arguments are attempts to show that a conclusion must follow from a set of premises or hypotheses. A deductive argument is valid if the conclusion follows necessarily from the premises.

Aristotle, the first person we know who wrote down laws of deduction, gives this example of deduction:

  • All men are mortal.
  • Socrates is a man.
  • Therefore, Socrates is a mortal.

The first two statements are called "premises". The last statement is called the "conclusion". The conclusion is taken from the premises.

  1. If the conclusion is wrong, then at least one of the premises is wrong
  2. If one of the premises is wrong, the conclusion may also be wrong.

Deductive reasoning is often called "top-down logic", whereas inductive reasoning is called "bottom-up reasoning".