Deduction is the logical process of drawing specific conclusions from general premises. In a deductive argument, the conclusion follows necessarily if the premises are true and the reasoning is valid.
A familiar example is: all mammals breathe air; whales are mammals; therefore, whales breathe air. The first premise states a general rule, while the second identifies a particular case. Applying the rule produces the specific conclusion. Deduction is often described as “top-down” reasoning because it moves from a principle or rule toward an instance.
Deduction differs from induction and abduction. Induction generalizes from particular observations, such as seeing many white swans and proposing that swans are generally white. Abduction starts with an observation and proposes the best explanation, such as inferring rain from a wet street. Neither guarantees its conclusion in the way a valid deduction does.
A common mix-up is confusing validity with truth. A deduction can be logically valid even when one of its premises is false; validity concerns the structure of the argument. When the premises are true as well, the conclusion is guaranteed to be true.