Deduction is a category of argument that falls within the study of logic. Put simply, deduction is a form of argument in which a conclusion necessarily follows from the premises. The following are examples of deductive arguments:
Premise 1: All dogs have four legs
Premise 2: Fido is a dog.
Conclusion: Fido has four legs
Premise 1: All golfers swing a golf club
Premise 2: Tiger Woods is a golfer.
Conclusion: Tiger Woods swings a golf club
In both examples, the conclusions are certain based on the premises. However, keep in mind two concepts when dealing with deduction—validity and truth. If an argument is structured in such a way that the conclusion is certain based on the premises, we say this argument is valid. It is valid whether the premises correspond to reality or not. However, in order for a conclusion to be true, both premises must correspond to reality. For instance, Example 2 above is both valid and true. It is valid because the conclusion necessarily follows from the premises. If all golfers swing a golf club and Tiger Woods is a golfer, then it must follow with certainty that Tiger Woods swings a golf club. The conclusion is also true since both Premise 1 and Premise 2 correspond to reality.
The following is a deductive argument that is valid but not true:
Premise 1: Every person in the United States Army shoots a gun.
Premise 2: Sharon is in the United States army.
Conclusion: Sharon shoots a gun.
In this example, the conclusion necessarily follows from the premises; thus, it is a valid argument. However, this conclusion is not true since Premise 1 is not true. It is not true that every person in the U.S. army shoots a gun. Some people in the army are chaplains, photographers, and doctors and do not shoot guns. Thus, although this argument is valid it is untrue because Premise 1 does not correspond to reality.
Deductive arguments are often looked to with favorability because they offer certainty. One criticism of deductive arguments, though, is that they do not produce any new knowledge; they appear to codify what we already know.