> Modus ponens is a valid argument of the form: If p then q. p.
> therefore q. In other words, the conditional <If p then q> is asserted to
> be or accepted as true, and the antecedent of the conditional <p> is also
> asserted to be or accepted as true. From this it follows that <q> is also
> true. (Modus ponens is sometimes called ``assertion of the antecedent.'')
>
> Modus tollens -- also sometimes called ``denial of the consequent'' is a
> valid argument of the form: <If p then q> and <not q>, therefore <not p>.
I should have added that there are two related fallacies: denial of the
antecedent and assertion of the consequent.
Denial of the antecedent: If p then q. Not p. Therefore not q. (To see
that this is a fallacy: If something is a Chevy then it's a motor vehicle.
This thing before me is not a Chevy. Therefore it's not a motor vehicle.
-- But is's a Plymouth.)
Assertion of the consequent: If p then q. q. Therefore P. (To see that
that is a fallacy: If something is a Chevy then it's a motor vehicle. This
thing before me is a motor vehicle. Therefore it's a Chevy. -- But it's a
Plymouth.)
Lloyd Eby