Editing Soundness (section)

== Definition ==
In [[deductive reasoning]], a sound argument is an argument that is [[Validity (logic)|valid]] and all of its premises are true (and as a consequence its conclusion is true as well). An argument is valid if, assuming its premises are true, the conclusion ''must be'' true. An example of a sound argument is the following well-known [[syllogism]]:

: ''<small>(premises)</small>''
: All men are mortal. 
: Socrates is a man.
: ''<small>(conclusion)</small>''
: Therefore, Socrates is mortal.

Because of the logical necessity of the conclusion, this argument is valid; and because the argument is valid and its premises are true, the argument is sound. 

However, an argument can be valid without being sound. For example:

: All birds can fly.
: Penguins are birds.
: Therefore, penguins can fly.

This argument is valid as the conclusion ''must be'' true assuming the premises are true. However, the first premise is false. Not all birds can fly (for example, ostriches). For an argument to be sound, the argument must be valid ''and'' its premises must be true.<ref>{{Cite book|title=Introduction to logic|last=Gensler, Harry J., 1945-|isbn=978-1-138-91058-4|edition=Third|location=New York|oclc=957680480|date = January 6, 2017}}</ref>

Some authors, such as [[John Lemmon|Lemmon]], have used the term "soundness" as synonymous with what is now meant by "validity",<ref>{{Cite book |last=Lemmon |first=Edward John |title=Beginning logic |date=1998 |publisher=Chapman & Hall/CRC |isbn=978-0-412-38090-7 |location=Boca Raton, FL}}</ref> which left them with no particular word for what is now called "soundness". But nowadays, this division of the terms is very widespread.