> I had occasion to be reminded of my unfamiliarity with some of the
> terminology logicians use -- and that philosophers use when discussing
> logic. I once figured out, by carefully reading posts on this refloector
> and others what modus ponens meant, but I have forgotten. Then while
> reading a paper on Popper, Falsifiability and Evolutionary Biology (Biology
> and Philosophy 11:161-191, 1996) I came across modus tollens.
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>.
Lloyd Eby