An argument form is valid if there is no substitution instance which renders the premises true and the conclusion false.

An argument is valid if its form is valid.

An argument is sound if

1. it is valid, and
2. all its premises are true.

An example of a valid argument form:

	If A, then B.
	If B, then C.
	If A, then C.

An example of a valid argument:

If Lee is Martian, then Lee has antennae.

Lee is Martian.

So, Lee has antennae.