# Validity vs. soundness

In logic there's an important distinction between validity and soundness. A logical argument or syllogism is valid if true premises always lead to a true conclusion. Validity refers to the structure or form of the argument and not to its contents, while soundness considers the structure and content. An argument is sound if, **and only if**, the argument is valid **and** all of the premises are true.

## Examples

Consider this logical syllogism:

- P1: All
**G**are**S** - P2: All
**S**are**D** - C1: Therefore, all
**G**are**D**

The form is valid and this particular syllogistic form is named "Barbara". If P1 and P2 are both true, C1 must be true. If we insert some "common knowledge" content into the argument, we can demonstrate an argument which is both valid and sound:

- P1:
(G) am a**I**(S)**man** - P2: All
(S) are**men**(D)**mortal** - C1: Therefore,
(G) am**I**(D)**mortal**

What happens when the premises are untrue? Consider the following example:

- P1: All
(G) are**toothpicks**(S)**made of metal** - P2: All
(S) are**metal objects**(D)**toasters** - C1: Therefore, all
(G) are**toothpicks**(D)**toasters**

We can prove that P1 and P2 are false by finding either a toothpick which isn't made of metal, or a metal object that isn't a toaster. In this particular case, P1 and P2 are not only false, they directly contradict each other (if all metal objects are toasters, clearly toothpicks can't be made of metal) and no external verification is required - the argument is valid, but the conclusion is unsound.

Let's look at an example where only one of the premises is untrue:

- P1: All
(G) have**mammals**(S)**backbones** - P2: All
(S) have**creatures with backbones**(D)**scales** - C1: Therefore, all
(G) have**mammals**(D)**scales**

In this example, P1 is true, but P2 is not. This one false premise renders the argument unsound. Let's modify this latest argument just a bit to demonstrate an important point:

- P1: All
(G) have**mammals**(S)**backbones** - P2: All
(S) have**creatures with backbones**(D)**three bones in each ear** - C1: Therefore, all
(G) have**mammals**(D)**three bones in each ear**

P1 is still true and P2 is still false (there are vertebrates with only one bone, the stapes, in each ear) however, the conclusion (C1) in this example happens to be true. If an argument is unsound, the conclusion may be either true or false - there's simply no way to tell from the argument alone. This issue is seen in many common logical fallacies and can be confusing to those who aren't skilled in assessing logical arguments.

It's possible to reach the correct conclusion by accident, but in order to actually demonstrate that the conclusion is true, the argument **must** be both valid and sound.