Rules

1. A deductive argument is valid only if its conclusion follows necessarily from its premisses.
2. The fact that a deductive argument is valid does not imply that any of the propositions in the argument are true.
3. If the premisses of a valid deductive argument are true, then the conclusion must be true.
4. In an invalid argument any combination of truth values for the various propositions may occur.
5. An argument is sound if and only if it is valid and has true premisses.